Um...quick decision: no, this is not a birds and bees conversation. This is hungry mouths one.
"Well, mummy works hard to give us a nice life and so I can't look after a baby."
X clearly think I am being a total wimp about this:"but I'll look after the baby and you can look after me."
'Mmmm, but a baby isn't quite the same as a doll, is it?"
She looks unconvinced. I remember my mother fostering several children all at once and me having a real life baby in my dolls pram so I totally can see where she is coming from.