- Tony dealt with stress by secretly acting like an 8 year old.
- Coulson found out first when he went to the lab and saw Tony putting away toys. The others find out in various ways one by one. Natasha is the last.
- Clint and Thor build him a massive play structure/fort in the gym.
- Tony and Steve paint together.
- It ends with movie night.
I'm pretty sure it was a well known author. On AO3.