When kids are hungry or tired they get really cranky and stubborn. A little food or nap would cheer them right up, but they angrily refuse the food or nap.
If you childless wonders think the answer is to give them snacks or naps before they get grumpy, keep in mind that when they're happy, they're playing, and they refuse to stop playing in order to eat or nap. You can put the food in front of them but they'll ignore it. Call them to nap and they refuse to go.