K.S.
S.:
I too am a SAHM doing some babysitting over the summer for a neighbor. It too will be part-time, for 2 school age kids and 1 preschooler. I am propsing to charge $600/mo or $150 a week for all 3. Less for the weeks 4 of the kids (school aged ones mine and hers) are up at the park for a playground program for the majority of the time. I didn't charge a whole lot due to the fact that I am unlicensed and my neighbor only works part time so it needs to be worthwhile for her too. Plus I get the added bonus of built-in playmates for my kids.
If you are enjoying what you are doing and it is not taking over your life (this was an important consideration of mine when making my decision); stick with the $100 a week. $400 a month is nothing to scoff at (especially if it's cash). That buys a lot of groceries, or diapers, or here's an idea: save it up and take a flight to a spa resort at the end of the summer (give your family something REAL to complain about).
K.