I've never heard of restricting a child's attendance to a max number of hours within core business hours. Surely if you open at a given time and close at a given time, if someone is paying for a full day it makes no odds who is present for the whole day and who is present for part of it.

I officially open at 7.30am but have two who are military who drop at 7am due to the hours the base works. I close at 4.30pm and don't extend that. But when my hours were longer, as long as nobody arrived before I opened and all were gone at closing time, then I was open for business.