I am not in the area, but my take on the difference in rates:
MH - are you considering a high school student or a college student? for HS, I'd go a little above minimum wage, so $8/hour. For a college student, maybe $10/hour. You're basically trying to attract kids who want to have a job but would rather not do retail or food service.
Sitter - High school, $10/hour 40 hour minimum. College, $12/hour, 4 hour minimum. Adults should set their own rates, likely around $14 and up
Nanny - a FT temp situation might pay a little more per hour, say $16/hr instead of $15. I would guess getting into a share would be tough unless you didn't disclose that you'd be switching to daycare - however, for a share you'd likely be looking at $10/hour per family.
<cue the posters claiming nannies work happily for either $8/hour or $35/hour, and GO!>