If you watch friends kids for free, should they provide the food?

To add a little context, my husband and I do not have kids of our own. We also do not make very much money. Our friends asked if we could take their kids for the weekend. We love the kids and agreed. Then our friend sent us a grocery list of things they will eat and what we should buy. Apparently they are picky and basically live off chicken nuggets, Mac and cheese, juice (amount a few other things). We try not to keep anything unhealthy in the house, but I understand kids can be picky. We have a pretty strict grocery budget and only buy exactly what we need for the week. I try not to waste money or food. I know adding these things will make us go over budget. It isn’t going to break us, but I will have to redirect funds from other things. Should they provide food or some money for the weekend? Would it be rude to even ask them to?