I think I would be worried and concerned for my children as a natural reaction but I don't think I would interfere with the school. I would assume there would be some information disclosed to the new school and to be honest, I don't know that saying something would get you anywhere as it is confidential information - I think it would also compromise the little girls privacy as well as the 6 year old boy.
It's not appropriate behaviour by any stretch of the imagination, but how can you not feel sorry for a 6 year old that has obviously been exposed to such behaviour as to think this is okay? You mentioned you saw some things in their parenting that you didn't like & you know they are known to DCP and you cut ties - not placing any blame on you however people turning a blind eye fails the child immensely. Maybe he needs adults who know right from wrong to be his voice where he can't?