Final chapter, sorry I took so long to post it here, you should follow me on ao3!
Crowley and Aziraphale looked at each other before looking back at William.
"William, my dear, what do you mean by 'switch places?'"
"Like go in each other's place. Holy water doesn't hurt angels, does it?" Aziraphale shook his head at the boy. "And hellfire doesn't hurt demons?"
"No." Crowley frowned, wondering where William was going with this.
"Why not go in each other's places."
It may have been the fact that the day had been far too long, but neither Aziraphale nor Crowley were following his thought process.
"What exactly are you saying?" Crowley asked wearily.
"Change places. You know, like that one movie we went to see a long time ago, where the mom and the daughter kept fighting so they had to change places, the mom was the daughter and the daughter was her mom."
"My dear, what on earth are you talking about?" Aziraphale tried to keep patient, but even he had his limits.
"I think I understand, angel. I believe our wonderful, brilliant, intelligent, little boy just solved our riddle."
...
William was left alone in Crowley's flat. His dad's had thought it would be for the best, as neither of them wanted a repeat of what happened the day before, and to be perfectly honest, neither did he. He wasn't really sure what he would do to pass the time, but luckily his dad had cable, and apparently had every channel in existence. Even some American channels. But even so, he couldn't help but think of his parents.
He had fallen asleep the night before while his dad's talked about how they would change places, so he didn't get to watch, but when he woke up that morning, he found Papa had left earlier to pop over to his book shop. Dad had wanted him to know he was heading out, only something was odd about him, rather the way he had bid farewell and walked out the door. It was almost as if he wasn't used to having long legs... It was just a very subtle difference.
He spent the time mostly exploring the flat, he noted that there were a handful of exotic plants. They were lush and green, and in fantastic health. He noticed there was a irrigation system keeping them watered, and that was really cool. These plants seemed more tropical than the ones back home, though. He recognized most of them; as part of his garden time was also learning about various plants from around the world.
"Oh dear! One of your leaves has a brown spot." He said to himself. "That's not good, we need to take care of that." Suddenly ALL of the plants around started trembling, which was certainly an odd thing to see. It was as if they all were suddenly terrified of him. William shrugged. Given everything else he's seen, frightened foliage was a drop in the bucket. He went into the kitchen wondering if there were any scissors.
...
Later on, probably a few hours or so, after William had fixed each plant, ensuring that each one was healthy and green, there was a ringing coming from the other room. It had a opulent throne in the center, in front of the desk. It was dad's office where he had his showdown yesterday. He found a cellphone on the desk, it looked expensive. The ringing was an alarm. He picked it up, hitting the dismiss button. When the alarm screen went away, there was a message on the phones screen: [When alarm rings, come to St. James's park, south of Duck Island.] With a link to directions to get there from the flat. He wondered if it meant his dads had finished with their bosses?
William followed the directions to the park in question, keeping an eye out for anything suspicious, but nothing seemed too out of the ordinary. He overheard some conversations though, from people wondering about strange weather from yesterday. Something about raining fish near the m25? He didn't pay attention to them though, he focused on getting to the park and seeing his dads. He headed to where the message had said to go. From a distance he saw them, and he smiled. He raised his hand to call to them, and paused. Did they change spots? He could have sworn they were seated the other way around. He frowned for only a moment, then shook it off. He called to them.
Crowley raised his hand to wave, Aziraphale beamed at the sight of the boy. That was the only other thing on both of their minds when they had been in heaven and hell, was his well being. Thankfully everything went right. William hugged both his dads.
"So what now?" He asked.
"Well, we could go home. What do you think, my dear?" Aziraphale rested a hand on his son's shoulder. It was funny, before he never really had referred to him as that, originally thinking everything was to be temporary, not really looking past yesterday.
Crowley thought for a moment. "Can I tempt you both to a spot of lunch first?"
Aziraphale beamed, a joyful wiggle running through his body. "Temptation accomplished!"
William beamed. The three of them headed off to a restaurant, one of his choice. His eleventh birthday was certainly a strange one, but he wouldn't have it any other way. It certainly was the best birthday ever.
And little did an angel, a demon and their adopted son knew, the day HAD been nice, and for the first time ever, a nightingale actually sung in Berkeley Square.