Just tell them no, we have a contract and these are the rules we agreed on.

I totally understand that you want to accommodate but sometimes we just can't. We have rules set by the gov't just like they have rules set by us and we all need to follow them.

If you can accommodate and want too than go ahead. Don't let what the other parents might do or ask be an influence. Most parents won't even care or realize that xxx child is coming Friday instead of wednesday for example. Even if they do, they are adults and should know that just because one parent is doing something doesn't mean they can too. Sometimes life is a case by case situation.