I agree with mcclure especially when it is an older child perpetrating the behavior on a younger child and I would say the older child has learned the behavior ..
There are huge red flags when kids act out like this on other kids...show me urs and ill show u mine is normal but masterbating another child leads me to believe she is mimicing what someone else is doing to her...keep Ur eyes open n talk to her.mom if u can. Good luck
Hi worried_momii. It is normal for children to masturbate, and or explore body parts. It's nothing that you should really be worried about unless she starts to have sex, talking about it with other people, or masturbating with someone else even if it's a girl. Explain to her that masturbating can be bad, and that you really should only masturbate when you are getting ready for sex and that you are ready to take the responsibility that comes with it. If you have any questions feel free to ask.
Sincerely,
AdultHelper
By your post it sounded more purposeful than curiosity, I would speak to her mom as its possible someone has done similar to your niece, I wouldnt let them be alone together .Talk to the 3 year old and tell her she should come to you if anyone does it to her .