I agree with Alpha - I would give the mother a call this evening and apologize and explain that this is a transition for everyone INCLUDING your child. It is quite normal for your child to feel that way and not understand why another child is suddenly in THEIR home all day.

My own 5 yr old daughter and son still make comments at times that make me think they don't get that this is my job and I am not just inviting these little 'friends' over as my own playmates. lol she still asks me if I love her more than the kids who come here every day. It is hard on our own kids and your child is bound to voice their concerns at times. It is hard on them too.

I would hope that having a good heart to heart convo where you explain that it is quite normal for your child to have moments where they feel possessive is normal just like there will be times where their child will go through stages of which parent they like more, whether they want mommy to go to work or not, etc.