I would tell the family with the 9-year old that I cannot take the 9-year old. If she wants to have her children in the same daycare, then she will look elsewhere and you can fill the spot with the 17-month old and interview for the second spot. She may decide that she can find after school care for the 9 year old. If it is only for an hour a few days a month, I wouldn't imagine that having the children at the same daycare would be a dealbreaker issue.