Having a second contract to cover just the extended hours is a brilliant idea. That would enable you to clearly state what day this arrangement was for, what the extended hours were, whether or not you are feeding this child from your food or if it's up to the parents to supply supper, what the fees are but more importantly, it gives you the ability to give notice on just the extended hours vs your regular service if it gets too much.
While the parents having that one later finish might well be dropping off later for this one day, you aren't actually gaining time off as you will have all the other children anyway.