This is as good a time as any to open the discussion about privacy and curiosity. Kids have been doing this stuff since the beginning of time. It's nothing new.
Having these intimate discussions with our kids makes them learn to feel more at ease talking about their curiosities and endless questions about how the world works with you .. especially those embarrassing or difficult questions to ask. It's better that they come to you for information than getting it off the street .. or heaven forbid .. the internet.
I have already witnessed a couple of threads here, where youngsters are getting conflicting information about sexual activities and how easy it is to get pregnant. Encourage you child to ask YOU the questions they need to ask so they can get good and accurate info.
Talk to you your daughter while spending time together doing a favorite activity. You can watch her favorite movie together with her. Schedule a specific time if you have to. Strike the issue when you are certain she is in good mood. Offer friendship but remain the parent. Read more from http://www.ehow.com/how_5023523_talk-teen-daughter.html#ixzz1twCQy4c6.
Check with your local police department to see if they have an officer who goes to schools to talk about "Stranger Danger". This way it's something that happens at school too. They discuss not letting anyone touch their privates and how to also deal with strangers. Peer pressure helps.
You should defiantly talk to her about this asap . In a calmly way as a friend , let her open up to you and see if she tells you why exactly she let him do so . And to always more less figure out the intensions the little boy has so if it's needed you to keep him away from her , Now a day as you may know kids are doing things that shouldn't be done at there age . If she doesn't budge and explain to you why that happened then shouldn't let her hang out with the boy or atleast not when you aren't around the whole time. Good luck