This idea was haunting me for the past couple of days until I gave in.
This is also based on the fanon that the Justice League are terrified of the Bats and have very little information as to what actually goes on in Gotham in general which means with the right acting, you can convince some of the more gullible members of anything.
(And since plans to troll the Justice League were made out in the open, fellow Gothamites may have decided afterwards to help out and continue the bit whenever a member pops up in Gotham for whatever reason.)
Disclaimer: I don't own Batman. That goes to DC Comics.
A Return to Childhood
By Bilbo-sama
Meetings in the Watchtower, many members of the Justice League were disappointed to discover, tend to be really boring. Usually the bulk of it tended to involve buzzwords like 'synergy' and 'community outreach' that made people wonder where in the corporate totem poll Batman resided in since it all seemed to come to him so easily.
This time, however, everyone welcomed the distraction of the doors to the meeting room loudly opening. Nightwing and Robin entered with a small group of mostly masked children unwillingly trailing behind them.
They looked very familiar and were very unpleased to be in the child safety leashes the two heroes were holding if the unholy hissing noises coming from them were any indication. One small boy with a white streak in his hair was gnawing on his.
"Report," ordered Batman. To everyone's relief, the noise stopped. However this meant several League members started fidget nervously as the children unnervingly stared.
"Outsiders," said the blonde one in purple.
"Outsiders," repeated the others one by one as if echoing each other.
Robin just sighed and pulled out a small water gun from his utility belt. If anything, the sight of it only made the 'outsiders' chanting louder in defiance.
Batman's seemingly neutral expression remained unchanged.
"Yeeeaaaah...I have some good news and some bad news," declared Nightwing over the noise as he ruffled the hair of a toddler in a black sweater and received a growl in return, "the good news is that this is just temporary and it will wear off."
"What happened?" asked Wonder Woman, who seemed unbothered by the children. Rather, she looked as though she was about to coo at them from the safety of her seat.
"We were resolving a minor Arkham breakout," (which meant only a minimum of three C-list rogues got out, two B-lists, or just the Riddler or Two-Face and thus something Batman's proteges could handle without him these days) "when Signal spotted a confused child outside of the perimeter," began Robin who was very calmly prying off a fully suited one whose face covering fully hid their features climbing onto Robin's shoulders, the yellow bat on their chest a possible clue to their identity. The being hissed. It didn't sound human.
"What the hell was that?" someone at the meeting table asked in horror.
"Oh right," said Nightwing cheerfully, "I forgot not everyone knows. Kids in Gotham are a bit more...feral when they're small. They'll grow out of it."
"Outsiders," a dark-skinned boy in a bright yellow hoodie and matching domino mask peered from behind Nightwing's legs, "eat?"
"We don't eat Outsiders, Signal," Batman lectured which caused a few heroes to whisper among themselves in horror (what do you mean baby Gothamites eat people?!). He did, however, reach into his own utility belt and tossed what turned out to a lollypop which Nightwing caught, unwrapped, and handed over to the currently younger than usual vigilante. "Continue."
"Signal then found another one and then another and started following a trail that lead to a crazed magician de-aging people to about six months to five years old," Nightwing adjusted his grip to the leash that had the purple blonde attached who was now following the boy with white in his hair's example and gnawing on the leash. "He called for back up and luckily we had finished detaining Calendar Man so we headed to his location. Robin volunteered to stay behind to transfer custody to the police.
"He somehow knew the others were there," added Robin which then lead to disgruntled whispering of "cheater!" to break out, "And he immediately cast his magic in their direction. When I arrived, Nightwing was the only one standing because he was barely in the spell's range. Together we disarmed the magician and through some peaceful questioning - "
"I bite!" the toddler in the black sweater announced gleefully. Nightwing cooed.
"...Some questioning," amended Robin, "we learned that it will last a day at minimum."
"So what's the bad news?" asked the Flash warily as the blonde managed to break out of her binds. Robin intercepted her with the spray bottle, resulting in a horrible screech that sounded like multiple demons screaming in pain.
"I was also hit with the spell," Nightwing pointed out, "but since I was further away from it, I will also become younger soon. Like, any minute now."
"...Ah," said Batman as if realization dawned on him, "Have you warned Agent A of this?"
"Of course, B. In fact he suggested we come here. Since y'know, he's just one guy."
"Old," chorused Nightwing's siblings like the horror movie children they are.
"That too," Nightwing reluctantly agreed. "And uh, I didn't arrive in Gotham until I was eight so I don't have the same instincts as the others."
"No chandeliers," ordered Batman suddenly.
"I make no promises," Nightwing smiled cheekily.
His form seemed to start wobbling. The children cheered and started to chant, "Change! Change! Change!"
"Well, looks like my time is up," Nightwing then saluted as his body shined a strange light, "good luck. Hopefully this place is still standing when we all change back."
The light flared and everyone shielded their eyes. When the light cleared, the Justice League looked back and found a child wearing a smaller version of Nightwing's suit.
The blonde in purple was in the process of freeing her siblings. The boy with the white streaked hair stole Robin's water gun and was holding him at gun point. The toddler in black was mimicking the child with the full faceless mask who was slowly twirling around in place. The Signal just untied himself from Nightwing.
Nightwing giggled ominously as Batman carefully rose from his chair as if he didn't want to startle a wild animal.
"I will count to three," he began.
"Scatter!" shouted Nightwing as several feral child ran in all directions, the Signal vanishing while doing so.
The Justice League stared as Batman audibly sighed and sat back in his chair.
"Right. Where were we?" his tone sounded like a defeated parent for a split second before professionalism took over.
"Aren't you going to catch them?" asked Aquaman.
"No," said Batman.
"Why not?"
"Because I had alerted probationary members of the League the moment I received an alert from Agent A. Let them handle it."
The members already at the table blinked.
"Unless you want to give it a try?"
"Nah," decided the Green Arrow, "some of them look like they need humbling."
"A wise decision," agreed Robin, who hated how a few of the newcomers treated him like a child instead of the experienced vigilante he is.
"...Alright," Superman decided reluctantly once the grumbling among the table eased. "Not our problem then. For now."
-ALTERNATE ENDING-
The light flared and everyone shielded their eyes. When the light cleared, the Justice League looked back.
Nightwing had gotten slightly smaller and lankier, his uniform thankfully shrunk with him. A pimple was visible on his greasy chin.
The blonde in purple was in the process of freeing her siblings. The boy with the white streaked hair stole Robin's water gun and was holding him at gun point. The toddler in black was mimicking the child with the full faceless mask who was slowly twirling around in place. The Signal just untied himself from Nightwing.
"You appear to be fifteen, Nightwing," Batman observed.
"Damn," sulked Nightwing, his voice cracking.
"I trust you and Robin will do your best keeping your siblings in line," Batman said with finality.
"We will not fail you, Father," Robin solemnly declared.
The boy with the water gun tossed it onto the ground and shouted "Scatter!" He and the other now wild children ran in all directions, the Signal vanishing while doing so.
Nightwing, with resigned responsibility, picked a direction to follow, futilely cursing the magician that started this mess under his breath while doing so.
(He was unaware that Batman had already called the probationary members of the Justice League for back up.)
The affected batkids (and other victims of Plot Device Guy) did retain their adult minds. They thought it would be funny to prank the Justice League and by stroke of good fortune Batman went along with it.
I had this vague memory of watching Justice League Unlimited and there was a guy named Vigilante at some point and that feels like the laziest name in the world. Turns out he's real according to tvtopes. He will soon meet his greatest challenge yet: tiny possibly demonic ninjas
Alternate ending Nightwing: Ah yeah I'm gonna get my vacation. *spell kicks in*
Alternate ending Nightwing: *is now slightly younger and thus can't pretend to remember nothing about being a civilized adult* Damn it.