Serena,
Neccessity is the Mother of invention!!! You are a genius. I think your prices are fair but a little confusing, especially for an "in a pinch kind-of mom" I would offer a "see me for specials" and just leave it at that with a flat fee, but definitly discount for multiple children. Don't discount yourself though, this is hard work and you deserve *at least* 7-10 bucks an hour. I would also offer "crafts" and "nutritious snacks"
I would use you!!! I'm a stay at homer and man oh man I would like to get out sometimes without strings attached....(my mother lives with my husband and I, so you can imagine, she'll watch 'em, but not without a little bit of a guilt producing speech beforehand) God bless your venture. T.