How to set rates?

So through my painting down at the market I have had two enquiries about party jobs.

The first one is actually my hubbies cousin, and I want to give her a deal, but I still want to cover my costs. There will be 8 kids aged 3-8.

I'm the only facepainter in PG I think, but it's not exactly "high income" up here. I know that some of the rates in the US are pretty high, but I have absolutely *NO* clue on what to charge for parties :s

Should I charge per child? per hour?

I think the closest to me is Shannon, and I think she's charging $100/hour? (Shannon would love to hear from you for suggestions on rates)

Also cause I'm just starting out I know I'm slow :s well, slower than some people :p I can still turn out a pretty good face in under 5 minutes though.

I was thinking at starting at $75/hour for ten or fewer kids and then up from there?

So confused :s

Trina