1. (the man is wearing an undershirt)?
2. If not, what about if she is wearing a tznius bathing suit?
- In general there is controversy among the poskim from which age a girl has to be dressed with tznius. Some poskim say from age 3 (Mishna Berura), while others say from around 5-6 or even 7. An eight year old girl is definitely of age that she may not appear in an immodest way. Regarding swimming, going swimming with a niece is the same as going swimming with your neighbor’s daughter. A man may not look at the covered places of his niece the same way he may look the covered places of his neighbor’s daughter. The fact that he is wearing an undershirt doesn’t protect him from what he is not allowed to see, and the improper thoughts that the swimming together can provoke. Therefore they should not go swimming together.
- If she is wearing a robe, it is better, but it is still not a correct thing to do. First of all, because she should not be swimming with men, as she is already the age of chinuch, and she should be taught that girls don’t go swimming with men. Secondly, if she is wearing a robe, it can easily come up in the water, and if she wearing a body suit, her form is still very noticeable, and you should not be looking at her that way. Therefore, while I can’t say that it is outright assur, it is definitely not appropriate, and should not be done.
Please see the following post https://dinonline.org/2010/07/12/mixed-swimming-for-children/