5 yr old daughter keeps masturbating with 6 yr old neighbor girl is this normal?

About a month ago i caught my 5 year old daughter masturbating with the 6 year old girl next door.  The girls mom asked her daughter (after i told her i found them masturbating together) what they were doing? how often?  It apparently happens everytime they are together. They don't touch each other they each get a pillow, stuffed animal and rock on it. The 6 year old said my daughter had showed her how and would hit her of she said she was going to tell me or her mother.  We told them not to do this with each other and I thought they weren't but i found out tonight that they have done it a few more times together.  is this normal?  i know my daughter hasn't been abused in anyway but it concerns me that she would hit her friend.  What should i do?

[Liquid4444]

It is normal for a child to masturbate, but not in a group. It is a private thing. Your child's play time with the 6 year old should be supervised. If left unchecked, touching MAY be next. They should learn to play together without masturbating.