My thoughts By Kathy

My initial thought was that there should be some kind of professional boundary between you and your cooperating teacher. I can see all kinds of reasons you wouldn't want to get involved in a situation like this. (For instance, what if there is some kind of problem with her children?)

I would just thank her for trusting you enough to watch her children and say you really don't feel comfortable doing it. (or don't have time to do it, have other plans, or whatever you can truthfully say)

As far as doing it without pay?? That's not appropriate for her to be even asking you in my opinion.