Possible trigger warning ...I could do with some advice

Trying to cut it short , we have a street family,  5 families in our cul de sac who made friends and kept each other company over lockdown. The one mum of adopted boys 6 and 9, I became especially close to . In part because oh boy , she's a Saint,  those boys have major issues that weren't disclosed before she took them on

Tonight they were all playing in the street as they do , and my girl dd6 and her boy ds9 disappeared for a very short while into our other friends drive. 

I was bathing dd6 and she suddenly says 'mum,  did you say nobody's allowed to touch my privates?'

That's correct... it transpires that my friends son who is thankfully very underage for 9 , had tried to pull her dress up, and prodded her once in that area, over her knickers.

I talked to dd In a very open way to assess what happened. She wasn't scared or frightened , and he didn't hurt her. She was literally just confused  I've reinforced that nobody touches you there, if they try to run away , tell an adult ! 

Now , how do I tackle it with the boys parents tomorrow??