Seriously, PP? Your model for reasonable and fair employment is Wal-Mart? For real?
Anyway, OP, there are two main reasons for part-time pay:
1) attract career nannies whi will be reliable
2) avoid having to find someone new when the nanny finds a full-time position
How many hours do you work now? Will you be looking for another job to supplement, or will you be cutting back expenses to make do?