If your child has abused another child, it may be that they themselves have experienced, seen or heard things that have disturbed his/her emotional and sexual development. Offer him/her the option to talk to a child therapist and accompany him/her if he/she agrees to it.
It would also be wise to consult —either alone, accompanied by your partner or with the family— a therapist to think about how your family operates and to understand why your child has not learned what is fundamentally prohibited.