Nice to see you back, and that things have worked out for you.

What were you thinking of changing your hours to? If it is just a decrease of an hour or so (say 8-5ish), then I wouldn't be too concerned about them wanting a change of rate. You could probably get away with a 7- 8 hour day for the same rate too.
However, if you want to drop to less than that, I would probably talk to the parents. I would think that once mom has gotten into a routine with the two kids, she may even drop back on the hours herself, but maybe not.

Good luck with whatever you decide to do.