What is the proper way to go respond to a possible molestation of a child?

my girlfriends 4 yr old boy just informed us that him and his friend another 4 yr old were touching their rearends with their penises the 4 yr olds mother has been known for years even the grandmother , we have never heard anything of this nature then while questioning our son he said its ok for grownups to touch like that ,we are about to blow a gasket at the moment because our boy has stayed the night at the others home several times and i know going down there and kicking the living **** out of every adult in the vacinity probably isnt the the best thing to do although it is sounding better every minute