There are sexual abusers out there that are children as well, if you suspect or find out that your child is sexually abusing another child, it is important to practice the following.
Take steps to ensure that the other child is safe, and is never alone with you child again.
Speak to you child immediately, letting them know that certain behaviors are unacceptable and they will receive serious consequences if they continue.
Reach out to professionals who can help you and your child. If your child is sexually abusing others, there can be multiple different reasons why. Getting help from professionals like therapists or juvenile sexual offenders experts can help you figure out a treatment plan that best fits your child.
Report to authorities to ensure that you, as the caregiver of the abuser is taking special precautions to never let the sexual abuse happen again.
Ask yourself "how did my child know how to do that?" or "why are they behaving like this?" Children who are sexual offenders are usually dealing with something that is effecting them greatly. So make sure that they are safe from abuse as well.