I would charge more than normal hourly equivalent for extended hour if I was going to do it. During business hour, I have all children paying that rate so I not earning "fees divide by hour open" but "fees divide by hour open times all children". That would by my hourly rate.

My fees = $38 day. My hours = 7.30am to 4.30pm which 9 hours. We allowed 7 children but take 6. My hourly rate would be $4.22 each child so $25 an hour.

I have two come at 7am and parent each pay extra $10 a day to drop 30 min early. Their contact say care available 7am to 4.30pm for $48 day. This make clear that fee is extra even if not here as rate was given for having two children both come early.