The biggest issue I see is that the parents of the b/a kids may be expecting you to be available to them when the children have a PD day, school closure due to snow, winter/spring/summer breaks. If you have the other children during the day then you wouldn't be able to take the older kids too.

I do know people that have done the school age but offered drop in for families needing a day to go shopping, the doctor, dentist, etc and couldn't take their kids always staying within ratio.

There are also a lot of caregivers that use the school age as a transition back to the workforce - generally it means their own kids are in school full days and they get a job that allows them to work part of the day. Many restaurants for instance love to have extra people work the 10-2 shift over the lunchtime rush.