Janet Rosenzweig, author of The Sex-Wise Parent and vice president for programs and research at Prevent Child Abuse America, wrote this for the "Healthy Kids" blog on Philly.com.
Parents often wonder about sex play between kids. For example, touching between 2-year-olds is very different from touching between 12-year-olds. But there is one universal indication for concern: If any coercion was used by either child to gain compliance from the other, adult intervention is imperative.
There is also one universal rule for parental reaction: Regardless of what you see, stay calm and think before you say anything to your child or another parent. If you scream, that can stay with the children long after any memory of the activities that caused the reaction.
Many families allow full nudity among their kids, letting younger ones bathe together. A child will let you know when she has outgrown this, and you must heed your child's request.
Toddlers may reach out in curiosity to touch another child's genitals, but this should be treated calmly, as though your child were annoying someone with an unwanted touch anywhere.
It is not uncommon for an older child to be curious and inspect a younger one, who may not mind at all. This seems to have been the case in the family of actress/writer Lena Dunham. Although some pundits labeled her a sex offender when her memoir revealed she'd inspected her little sister's genitalia, it apparently was a nonevent for her sister. Parents who come into this situation might calmly announce that playtime is over and then speak with each child in private. That conversation should be about respecting the personal boundaries of others, and offering to answer questions about body parts. Doing this without shaming the children is crucial.
If one child seems scared of the other, pay close attention. Speak to the child who may have done the coercing and try to discern the motivation.