Need childrens books on adoption? (pre-school age)?

[kennebunklmt]

My girls were in foster care and have been adopted. My oldest B is 4 (she's been with us since 3 months) and my youngest L is almost 3 (she's been with us since 9 months). They don't remember their parents (both are drug addicts and know that if they 'clean up' they can see the girls, but it has yet to happen). The only life they know has been with us- although they remember their adoption days because they were recent.
They know they are adopted but don't quite understand it yet. B recently asked to see a picture of her in my belly (we had a biological son and she sees those pregnancy pictures). I explained to her that she didn't grow in my belly, etc.
Today she asked who's belly she grew in and why wasn't it mine. I briefly explained it (her mother was sick, etc). She looked very confused!! Of course we will keep explaining it to them when they ask.

We have a few adoption books for children, but I would like more. I want some that show other children have been adopted. Not an educational book for me, but a real children's book.
We have 'over the moon' and 'little miss spider' but all of the other ones I found were about international or infant adoptions. Does anyone know of any good ones for pre-school age??

Thanks!