B.H.
What wonderful fun! My daughter since the age of a 9 has been "helping" out the new moms in the neighbourhood. She is now 12 and has many babysitting jobs because of it.
Her responsibility was to "play", play with the baby if the other kids needs a bath or to be fed, or play with the other kids while baby is being tended to. One mom was brilliant and she had planned play time with each child when my daughter came over (she has 3 kids).
My daughter earned $5 an hour and even though she said she would do it for free, I INSISTED she take the money. Children need to understand that their time is valuable, that they are "worth" getting paid. If as a parent you decide to have them use that money for something special, or to save it, that is up to you. It is not illegal to pay a child. It is actually encouraged. The earlier kids have an understanding of money the better.
It is very important for you and her and the new mom to sit down and have a quick discussion about expectations. How long will she stay? What are her responsibilities? Treat it like a "job" and teach her that she will be responsible, it's not just a time to play. She is only 9 after all.
Have fun with it!
B.
Family Success Coach