depends how old children are. my kids are still relatively young, and if they would see something not tznius – they would say in shocked voice, “ema, that’s not tznius.”
and in this case, i would just explain to them, “you’re right. so don’t look at it.”
if kids are older, and there own taavos are involved – can be little more tricky. can explain to older boys that they should use other bathroom like father does and explain why, or can explain to 30 year old girl, and ask her if she can put them somewhere else. expain: nothing wrong for her, just don’t want your boys seeing things that are not healthy for their eyes to see. if she refuses, then only choice is to work with your own kids or not go there again.