How should I handle a 4-year-old's curiosity about "private parts"?
Showing one's private parts to one another is quite normal behavior for the curious preschooler. Some bolder or more curious children may even reach out to touch their playmate's genitals. Preschoolers may also try to curiously touch a parent's genitals or a mother's breasts. However, if the behavior changes to mimicking sexual acts, or forcing or coercing another child into sexual acts, the behavior is no longer normal.
If your daughters seemed to be uncomfortable with the 4-year-old boy's sexual play, you may remind the boy that not all children like to show their private parts and ask him not to do it again. You should certainly let the boy's parents know that he is displaying some normal sexual exploration that is making some of the other children uncomfortable. Each family will need to decide when to set limits on sexual exploration based on their own values and tolerance for such behavior. It would be a good time for all parents involved to have a talk with their children about body awareness and safety in general. Preschoolers often respond well to being taught that "private areas" are the parts of their body covered by a bathing suit. They should be reassured that no one can touch their private areas except for their parents or the doctor when they might be ill or during a checkup.