This one is going to sound a bit strange, but if you just need a boost each week to help you out and are willing to be messed about a bit, consider employing a single parent or even two fo them.
Seriously, I know at least one other single parent on here that came into window cleaning for the same reason as me. There is not many jobs out there that employors are willing to employ someone for 16 hours a week, beween 10am and 2pm 4 days a week or so. This suits a single parent very nicely. Working Tax Credit really boosts the payments but they do need 16 hours a week.
The reason I say 4 days a week, its simple. Schools have a lot of time off, but they have mondays off more than any other time due to bank holidays so its best to employ tues-fri. It also means that if for some reason he has to pull out one day due to a teacher training day that he can make it back a bit by finishing a bit latter for a few days, or starting earlier, or heavens forbid, working the monday. If you think you coud be the sort of employer that would be willing to put up with a bit of wierd time keeping and would be willing to let a good employee make it back up then it could be worth it (ie you must be a bit flexible).
It might not be the answer, but if you want someone to work 4hrs a day for 4hrs a week phone the local jobcenter-plus and ask to speak to a lone parents advisor.