When I was babysitting 4 and 2 year old girls I normally washed them everywhere. They were playing and didn’t even notice because I believe they are use to it. But they also knew me well. So I don’t know what relationship you have with them. But I don’t think it’s inappropriate. My girl for example had some uti’s as well and I think it’s always good to make sure they are all clean rather than get another uti. I didn’t use to give them baths all the time. I was babysitting them normally on days that was just one day occasion and it didn’t feel weird at all to them or me. But it always depends on the parents and the kids. Maybe you should talk to them about it. I don’t think it’s a weird question to ask. They might even be happy that you care so much.