Is it normal for a 6 yr old to stick her finger in a 4 yr old's (both girls) butt, I am outraged over it!?

I have had the problem of this 6 yr old neighbor girl constantly taking my 4 yr daughter's clothes off, she has been told repeatedly to stop, even warned her if it didn't stop she would not be allowed to play with my daughter any more, well it happened again, only this time my daughter said the 6 yr old stuck her finger in her butt and that it hurt, I was outraged, I ran the little girl and her mom off and haven't let my daughter play with her since. I feel like my daughter was molested...Is this normal behavior??? I know my daughter is never playing with her again....Please help