is it permissible for a 6 yr old girl to be in a pool that her 25 year old uncle is in? The uncle is a lifeguard and plans to help child improve her swimming.
The fact that the man is the girls uncle is not going to matter here, because he may not see he when she is not dressed properly, just like any other man can’t.
Although technically it might be permitted, still it should not be done. There are different opinions as to what age a girl has to dress with tznius, i.e cover her legs, sleeves past the elbow. The Mishna Berurah says that the age is 3 years old, others say that it is later, approx. 6-7 depending on the child. If one wants to rely on the lenient opinion they may, however for her to go to a public pool that others there hold like the Mishna Berura, it is not fair for them. Secondly, even if technically it is permitted it is not tasteful and in the spirit of the halacha. Although a child of six may not be required to wear a proper dress etc., however appearing in a bathing suit in front of men is not in the spirit of the halacha.
Biur Halacha O:CH B75 D:H Tefach, Chazon Ish O:CH 16-8, Children in Halacha chap. 7 ftnt.4, Rav B. Forst shlit”a