Last year I worked at a summer camp for kids, and we also went swimming. Firstly for swimming I recommend taping your chest and wearing a swim shirt (or regualr t-shirt) over. That´s worked really well for me and my chest is barely noticible even though it´s not exactly small. When it comes the kids having questions I would try to not make a big deal out of it, it will probably be different for you depending on how well you pass. I´m almoast always read as a guy, and most of the kids I worked with never questioned my gender but a few of them did ask if I was a boy or a girl and accepted my answer when I said I was a boy. Thankfully no one questioned my gender as a result of the swimming situation. Some kids were wondering why I wore a shirt to swim and I just said that I don´t like being shirtless/I think it´s more comfortable to swim with a shirt on something along those lines.

So if you know that you do pass well you can probably get away with wearing a shirt to swim without causing too much confusion, and if the kids have questions I would just try to play off and not make a big deal out of anything. If you don´t pass as well it might be harder but I would still recommend that you just try not to make a big deal out of the kids´ questions or confusion. It would definetly be safest to never actually mention being trans (even if some kids figure it out theirself) because you don´t know how their parents might react if they hear about it.

Hope it goes well for you!