This is part of the reason I don't take SA.
But what if you tried a "drop in care" rate for SA during the school year say of $30 a day. (drop in rates are usually more) I wouldn't offer sibling discount during the school year. Then come say March if they want their school age to be guaranteed a spot for summer they have to pay a deposit of say I dunno $200 and sign a "Summer Contract" for SA and then charge them the $15 for the sibling discount. That would then guarantee they had a spot for summer and it would let you know if you had SA for summer or if you needed to find younger full time kids for summer.
This is just and idea. I'm try to help.
