One idea could be that the Avengers are initially shocked but then become very protective of Spider - Man. They might start to train him more carefully, like Tony Stark making sure his suit has extra safety features. Another could be that some of the Avengers, like Captain America, take on a mentor role, teaching him about responsibility and heroism in a more age - appropriate way.
Well, some of them might be really surprised. Tony Stark, for example, could become very protective. He might start making special suits for Spider - Man that are more suitable for a kid, with extra safety features. Captain America would probably be more concerned about the fact that a kid is out there fighting. He'd want to make sure Spider - Man is trained properly and not in over his head. And Hulk, well, he might not really understand at first, but then he'd probably see Spider - Man as someone he needs to look out for too.
The Avengers would probably be very concerned. They might try to keep Spider - Man out of dangerous situations at first. For instance, Iron Man could limit his access to the more dangerous missions.
Well, it could also happen that Peter's Spider - Man suit technology gives him away. Maybe the Avengers have some advanced tech - sensing equipment, and when Peter is around in his suit, it gives off some unique signals that the Avengers pick up on. So, they start to look into it and eventually figure out that Peter is Spider - Man.
The Avengers could find out who Spider-Man is in fanfiction by a trusted insider accidentally spilling the beans. Or perhaps a villain uses this knowledge to manipulate the situation and forces the truth to come out.
One big challenge could be that the Avengers might try to over - protect him. They might not let him go on as many dangerous missions, which Spider - Man might see as holding him back. Also, he has to deal with school and normal kid stuff on top of being a superhero. If his identity gets out among the Avengers, there's also the risk of it getting out to the bad guys too.
Another possibility is that in the fanfiction, Spider - Man's identity was revealed in a way that put the entire Avengers team at risk. The Avengers might then decide to distance themselves from him to protect their own secrecy and safety. This could involve kicking him out of the group to avoid any further potential threats associated with his exposed identity.
One idea could be that during a mission crossover, Spider - Man gets injured while trying to protect a civilian from an unexpected attack by a powerful enemy. Maybe he takes a hit meant for an Avenger and his suit's technology malfunctions, leaving him vulnerable.
In a fanfiction, Spider - Man could get lost in an alternate dimension during a mission. The Avengers then have to find a way to bring him back. While in the alternate dimension, Spider - Man meets different versions of the Avengers and has to fight his way through various challenges to return home.
Tony Stark would probably be both proud and a bit worried. Proud because Peter is this brilliant young mind who has managed to be a hero on his own, but worried because he's just a kid. He might start to hover over Peter more, offering him better tech and trying to mentor him more closely. Captain America would likely be very accepting. He sees the good in people and would be happy to have another brave soul on the team. He'd probably offer Peter some words of wisdom about using his powers responsibly. Thor might be a bit clueless at first. He doesn't quite understand the whole 'secret identity' thing as well as the others, but once he gets it, he'd be all for having Peter around. He might see Peter as a little warrior and be eager to train him in his own Asgardian ways.
Some Avengers might be protective. For instance, Captain America could immediately start giving Peter advice on how to better handle his dual identity and keep himself safe. He might offer training sessions to help Peter improve his skills as Spider - Man.