I would first consider her circumstances. Is that all she does for income?
If so, I would consider the idea.
If not, I don't think an occasional sitter needs a raise.
I was a nanny for 4 years and always got the same pay. They did however give occassional bonuses (once or twice a year).
If money is tight for you, bonuses may be cheaper in the long run. When I got bonuses they were $10 extra added on to my usual pay.
That may be a good compromise.