Do you have any kids that sleep longer than an hour? If not, I would consider having nap an hour long and than back to normal. Some kids only sleep an hour and it sounds like you have a whole group of them! I am like you, I find it more stressful to listen to them cry for the last hour and it's not a break lol. If it's only occasional I can handle it and would let them CIO but if it were every day than I would just make nap an hour.