My son told me that a little girl pulled his pants down and started playing with him. This is after he told me that she sexed him. I have spoken to her mother and she is acting like it's not a big deal. Upon further investigation it has happened to other kids that play with her. What should I do? Should I let it go, or involve CPS?

What does "sexed him" mean?

How old are your son and this girl? Where did this happen?

What does "sexed him" mean?

How old are your son and this girl? Where did this happen?
He's4 she is 6 and he told me that she played with him.

It happened outside

Well first you need to take this as an opportunity to explain to your son not to let anyone do that. And what to do if someone tries.

Second, at those ages its natural curiosity, but it does need to be nipped in the bud.

What do you mean "further investigation"? What did you do ans what exactly did you find out?