Instead of a morning nap routine there is no reason why you can't have a quiet time while the other child is sleeping. Use that excuse that you have to whisper and be quiet while little one is sleeping. Provide a space with lots of pillows, let each one have a blanket and some stuffed toys. Read a few stories to them, play some calming music, sing with them. Any that are really tired will soon rollover and close their eyes. The others will get the break they need. Most kids do not get enough sleep overnight to meet the guidelines depending on what time the kids are gotten up at home. Many do need another two hours to catch up. The trick is to do it early enough that they are still as you say tired when they come and it won't interfere with the afternoon. Any child that does fall asleep should be woken up after an hour. Follow the rest time with a drink and active play. Having a group rest so to speak will be a lot easier on you than trying to get them to lay on their own mats and stay there. Have them rest to listen to music and have a cup of tea and enjoy.