HYDRA didn't create only one magic-user while in possession of Loki's scepter. There's another, and his powers are... a little different from Wanda's. The Avengers crew breaks into a HYDRA base to rescue Tony, who was kidnapped three weeks ago. Only, he isn't the way he was when he left them to go check out the place. Instead, he's now five and a half years old. And he's absolutely terrified of Steve. The Avengers do what they can to keep little-Tony happy until they can get Strange to come take a look at him, and (hopefully) change him back to his rightful age. It's not so easy dealing with a traumatized five-year-old, though, especially one who has some major attachment issues. Toward Bucky, of all people. Throw in some HYDRA shenanigans, and it makes for an interesting time, if not the most peaceful. Older-Tony isn't going to appreciate how much little-Tony blabs, either, but it's pretty hard to tell a five-year-old to shut up when no one wants to hurt his feelings...
Imagine Tony getting deaged into a 5 or 6 year old, but instead of the team finding him, Bucky, still on the run and struggling with himself and the winter soldier, does. Bucky tries to take care of him, learning of all of Tony's issues and the abuse he went through. Eventually he has no choice but to come in and bring Tony to the avengers. After Tony comes back to himself they deal with the aftermath