Parenting

I have a 5 year old daughter that thinks touching a boys penis or any male’s penis is funny. And she wont stop doing it. (She doesn’t do it every day but she has done it to many times.) Their has been 2 incidents at her day care with this issue, I don’t know what to do anymore. She doesn’t see no sexual behaviors in the house because my husband has been away already for 2 years, and I live alone with my 5 year old daughter and 3 year old son. Maybe she got it from playing with her brother when I have finish taking them a bath, and she started to tickle him to the point to get to the penis. What should I do because today she went and touched her grandfather’s penis, and if course of dad put a stop to it quickly. What can I do to make her stop this.