11 year old touched my son

How would you feel if your husbands 11 year old daughter was touching your 4 year old son.  She has moved with her mom but he wants her to come back.  He very well knows how I feel.  I don't not want that girl around my son.  It's been two months since tho incident ? What should I do ?  If she wasn't 11 I'd chocked her !
Clarification;  she did it for over a month!  She also got caught because my son started screaming his penis was bleeding!  She's been caught having boyfriends also multiple of them.  She repeatedly stated she wasn't touched.  She's knows more than she needs too.  I brought her Into my home because her mother allowed her to wear make up have Facebook.  Took her on shopping sprees she's already dyed her hair! also was reported at school for talking about drugs !!  To help her and give her a better life I switched her school put her in dance.  Than she started failing school and left back to her mothers.  After all the sacrifice to go complain.  Than tell her mother my son treated her bad and looked at her wrong !