Girls start early don't they?

I would not have imagined having to deal with this at the Kindergarten level. And if it is any indication of what it will be like during the tween and teen years, so help me. Reagan had a little friend she met on the bus who happens to live down the street from us. They would play together fairly well. Sometimes the girl's 2nd grade sister would join them. Then something changed and I still don't know what. This little 5 year old girl would yell at Reagan during playdates (over nothing), Reagan would cry and I would send the girl home. This girl would tell Reagan at school that she is not her friend anymore. Fine, whatever. But then she would show up at the house in the evening to play. Yesterday the girl's sister told Reagan that her "friend" hated her. I asked Reagan if she were sad about it. She didn't really answer. She seems okay. I tell her she has other friends to play with and different activities outside of school. I tell her what a great person she is why she is a great person. I wonder if this bothers me more than her. This morning I watched after I dropped her off at school and she went up to this little girl, but the girl and her other friend ran off. Reagan just ran off to play by herself. So do I just let them deal with it themselves? Talk to the girl's parents (which might make things worse)? Talk to Reagan's teacher to see if she has noticed anything? Start bullying the kid myself? I know not to do that but is what I feel like doing lately.
Oh yeah--Camden brushed his teeth with toilet bowl water this morning. He was in the bathroom so I checked on him and found him doing that. Lovely.