If you observe the full Hijab, technically it’s permissible to go there. However, if such a mixed setting will put you in a position where you’d look at those male parents in their swimsuit, then it would not be permissible. You have to be careful and avoid looking at them. Also, you should not go in the water in the presence of those males. Even if you are in full hijab, getting wet in water in front of them is not appropriate