Ask other working mothers how much they pay for baby sitting such as what you will be doing. Get two or three answers, then sit down and discuss this with the parent(s) of these children. And I'd suggest that a signed statement that this sum is what is what all agree with.
And what about food, who pays what for that? Is either of them allergic to anything? Make sure that is spelled out.
What about emergencies? You should have some sort of signed statement allowing you to take them to a doctor till one of the parents can be located and come to where the child is.