S.K.
Savannah, I am so sorry for all the sorrow your family has experienced in the past year. I am glad to hear you are relying on your faith and family to help you through this very difficult time. Your children and the children you care for are blessed to have someone like you in their lives.
That said, it seems like you are putting up with more than you should from the kids in this new family. If one of these kids were hitting a child that was not your own and the parent complained--what would you do? Are you going easier on the hitter b/c it is your child being hit and you don't want to appear to over-react? My guess is that since this girl's brothers play roughly with her, that she is simply modeling what she has experienced by playing roughly with a child younger than she. However, the mother cannot use that as an excuse for her child hitting your newborn in the head with a train!! What if she hit the soft spot and caused brain damage?! As you have found, you can't realistically keep them apart or watch them both constantly and still provide good care to all the children. I would say put this family on two week notice that if you do not see acceptable improvement in the aggressive behavior of their kids, they will be out. Mom and dad need to reinforce your rules at their home or there is no hope the kids will stop being so rough. Good luck!