On the rare occasions I've been asked to do one-off things like this, I've charged an amount inversly proportional to how keen I was to do it - if I was keen, charge little, if I wasn't, charge a lot.
If these are 'real' friends and the kid's OK and you would like to do it and you have the time, charge a minimum amount, maybe just a few bucks an hour plus costs. If they are just aquaintances, the kid's a pain in the a** and you really don't have the time, I suspect 100 bucks an hour would put them off while not being unreasonable if compared with other professional services.
The other way to do it is think of something you want, really want, and would be prepared to spend a day or two of your time in exchange for getting it. How much is it? That's what you charge.
Hope that helps.