J.S.
Since you're seriously considering starting to sub in the Fall, I would decline the offer. It wouldn't be fair to the neighbor who likely won't be able to pay you a competitive wage and that's why she's asking you to babysit rather than going to a daycare situation or kindergarten enhancement situation. Not only that, but even if you started to do it as a favor and then started working in the Fall with something that pays better she'll end up in the same position she's in now.
Decline.