Some fans are devastated. Levi has a huge fan following, and his death in a fanfic can be like losing a real friend for them. They might stop reading the fanfic altogether if they can't bear the thought of him being dead.
Some girls might be a bit shocked at first. But then, most of them would probably be very accepting. They could start asking the crossdresser about their outfits and where they get their inspiration from.
Some girls might become more confident in themselves. They see it as a step into adulthood and a new phase of life. They start to take more control over their sexual health and relationships. For example, a girl could start to educate herself more about contraception and healthy relationships, feeling empowered by her new experience.
Eren would probably be very curious. He's always been eager to learn and understand new things. Meeting kids from another time would be fascinating for him. He might ask them a lot of questions about their world and how it differs from his. Levi, on the other hand, would be more guarded. He'd be suspicious at first, but once he realizes they're not a threat, he might start to warm up and show his more protective side towards the kids.
Some boys and girls might become withdrawn. They could be afraid to express themselves around their mother after being spanked. For example, in some stories, the child would avoid eye contact and stop sharing their feelings.
The boy could react in different ways. At first, he may be shy and keep to himself. But as the girls start having fun, he might loosen up and start laughing along with them. He might also feel a bit awkward when they start talking about girl - stuff like crushes, but he could still be polite and listen.
Some might panic at first. But then they could start to organize themselves. One girl might check for any possible ways to get out, like through a window or by fixing the door. Another might try to stay calm and keep the others from freaking out too much.
If it was an accidental or really strange situation, you can choose to laugh it off. But if it still bothers you, you can distance yourself from them. You don't have to stay in a situation that makes you feel uneasy. Maybe find new friends who respect you more. You could also talk to someone else about it, like a family member or another friend, to get some advice on how to handle the situation better.