My 9-year-old daughter is experimenting with her friends.

Mother: "My nine-year-old daughter recently came home with a bruise on her lip. She told me that she had been playing 'school' with some friends behind stacked mats at school and had been 'kissing' them.

It turned out they'd been lying on top of each other with a few girls still clothed. My partner and I aren't sure how to handle this. To what extent do we limit this, and where do we let it go?

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Belle Barbé: "There's nothing unusual about this behavior. In the case of 'kissing,' from around the age of seven, children can really start to feel affection for each other, and they show it in a way they know adults do. Lying on top of each other is one of those innocent playtimes we know as 'playing doctor' or 'mom and dad'."

There are a few things you can keep in mind. The games must be voluntary and fair. That means the children are roughly the same age and want to participate; no one is in charge. That bruise on her lip must have hurt a little. You could check that no one was forced.

"'Consent' is still a big word at this age, but you could talk to your daughter about it. For example, how she can tell if someone is enjoying a game like that. Is the other person enthusiastically participating, or is she eager to leave? And if she can't tell, she can learn to ask, 'Do you enjoy this too?'"

Also ask who she would go to if something happened that she didn't want. You could also say, "I understand you're curious, but make sure you don't hurt each other."

"Maybe the teacher should point out those stacked mats. Bad things can happen in a place like that, out of sight of adults."

Normalizing behavior

Daphne van de Bongardt: “It's good that you asked about the bruise, and that your daughter talked so openly about what had happened.”

This kind of experimental behavior is part of normal development at this age. Children practice all sorts of things all the time, including intimate behaviors like kissing and lying on top of each other. These behaviors aren't necessarily sexual at this stage, at least not in the way they might become later. It's a safe, giggly way of exploring with familiar peers, without the tension that comes with falling in love later on.

"In this case, setting limits is unnecessary and also impractical. During this age group, and the puberty that follows, children and adolescents spend more and more time outside the home without parental supervision. As parents, you then have more limited insight into and control over what they do."

What helps is open communication about what's on your daughter's mind and how she's experiencing it. Think back to what you did at that age, and talk about it with your child. For example: 'When I was your age, someone also wanted to practice kissing with me.' Or some other anecdote. This normalizes the experimentation. It also shortens the distance between you and your daughter. This can help her openly talk about her experiences, and you can offer some advice if needed. And she'll be more likely to speak up if something's bothering her.

Belle Barbé is an educator specializing in sex education and the author of "100 Answers to Sex Education." Daphne van de Bongardt is a professor of relationship and sexual development, education, and health at Erasmus University Rotterdam.