Giving mother in law money for childcare

When I return to work my mother in law will be helping with our baby. While I’m sure she would do it for free, we live in Hawaii and it’s expensive and my in laws could always use some help with money (father in law is retired, mother in law was working two minimum wage jobs to pay rent but may lose hours by watching baby). I’m curious what people think is a good amount for 3 days a week about 8-9 hours. Considering she’s family and wants to help but also giving her something for her time and care.

Thanks!