A/N: Written for Tony Stark Bingo square: It Wasn't Worth It. Title from Avengers: Endgame
Tony and Harley were tinkering in Harley's New and Improved Certainly Exceptional™ Lab, chatting about Science! as they were wont to do, when Harley asked Tony if his dad had taught him all this stuff.
Tony suddenly became very concerned with a laser… pointer as the boy looked at him curiously.
"No," Tony cleared his throat. "My dad didn't like me in his lab. He was more interested in finding Captain America and inventing weapons than being a dad or teaching things."
"Your dad knew Captain America?!" Harley squeaked.
"My dad made Captain America," Tony corrected, torn between bitterness and getting caught up in Harley's excitement.
"Really? So, he was around during World War II?" the boy clarified.
"Yup," Tony confirmed, scrubbing a hand down his face.
"I guess your dad did like making weapons," Harley remarked pensively.
"Steve's not like that. Not really," Tony concedes. "Besides, my dad quit the weapons business for a while, wanted to create flying cars and a world of tomorrow. Guns just became more profitable. The older I got, the more he branched out into bigger and more efficient ways to kill people."
"You call Captain America Steve," Harley was laughing at him in delight.
"Yeah, well, fighting aliens together really bonds people."
"I'll say," he calms down, still grinning. Then he continues connecting the circuits Tony had arranged for him. "My mom says she hates to work so much but knows it's how me and my sister eat and stuff, so that makes it worth it. Must've been like that for your dad, too."
Tony raises an eyebrow at the delusional kid.
"I mean-" Harley continues quickly, "when my dad left, my mom got more and more busy working and trying to keep the house and stuff. Didn't want us to miss out on having anything, even if it means missing her. She tries, but…" he shrugs, still staring down with Tony's attention fully on him. "I figure, your dad sees you, after living through at least two wars, and he knows he wasn't even able to protect Captain America? No wonder he didn't have time to be a dad. He was probably so busy making sure he could win any war you would have to live through. And probably forgot about you like... Anyway, at least you found Steve in the end, right? The Avengers, Earth's Mightiest Heroes," he starts cheering, making Tony crack a smile as he starts chasing the kid around the workshop, their tinkering devolving into an impromptu game of childish jokes and tag, just like it always inevitably does when Tony visits.
A few years later in the past, Tony meets a Howard Stark who is heading home, leaving his work at the office as he tells Tony there is nothing he wouldn't do for his child, and Tony sees a yet another nervous parent, terrified their child's world won't be any better than his own, willing to make any sacrifice to keep his kid safe.
And he understands.