OK, Just guessing I would have thought $10-$15 /hr

I got this from google, from an Australian babsitter indroduction site:

... Expect to pay between $12/hr (few responsibilities) to $15-20/hr (for experienced nannies during the day). Rates vary according to sitters' experience, the duties required .....
I think this is a good guide. If you kids are already in bed, and all the sitter is doing is watching TV in your lounge room, and checking on them etc then I reckon $12/hr would be good?