This sounds like a rough situation to be in. I am waiting for this to happen to me in the future and I don't know exactly what I will do or say. Like you I am "nice" and would want to help out because after all it is time with your kids.
I know I am too much of a push over and would wonder what I would do if the roles were reversed to a point when I would need her to watch the kids. Some one with more experience with these issues will probably have a well thought out plan for such occasions.
Could you make it his problem to line everything up and pay for the child care so you can do what you have planned? If it is his weekend it should be his reponsibility.
M:34 XW:34 Together: 10y Living: 9y Married: 7y Son:6 Son:4 Separated: 12/28/13 Piecing: 5/2/14 Separated 2nd: 10/16/14 W filed, but pulled it: 11/5/14 papers served: 1/27/15 D final: 3/6/15