Most parents have bills showing they paid for water, electricity, food, rent - and child support can reasonably go toward part of those expenses. After all, the kid needs a place to live, food to eat, utilities.
A general request for receipts could come up if you're asking for more money and the court or other parent is requesting proof you actually need more money. It is reasonable to ask for receipts if you ask for 3K medical and don't have proof the money was spent, or asking for $500 a month for kid activities without explaining where that goes. Ditto if you ask for X amount for childcare, and they want to know that a similar or larger amount went to a daycare or an afternoon child care program.
And it is no uncommon to see a request for proof of where all the money went if someone pays $800 a month child support yet the kid looks underfed, wears worn out clothes and seems neglected.