How frequently do you babysit? Is this a primary job or just getting a little extra?
If you do a few hours a week across different people you could probably raise your prices $5 across the board.
If you are consistently babysitting for the same people, many hours a week, you might have to determine if they can still afford services after a price increase. Especially if it's because they are working or something. I'd feel less bad raising prices if it's so they could go out on dates and such.