I have never gone back with the kids for naps. We first started going in 1999 when my DD's were almost 4 & 5 1/2. We didnt even take a stroller. The girls never got tired, whined, or complained. My DS was born in Oct 2001, and we took him in January. He just napped in the stroller, and again, we never had to leave to go back to the room. DS will be 4 next month and Thanksgiving 05 will mark his 8th trip. I have never taken him back for a nap. We go every January, and have gone once in OCtober and twice at Thanksgiving. The hours in January are so short, we would have missed out on so much. DS just this past January would fall asleep in the stroller when he was tired. It has always worked out perfect for us to stay in the parks and let him nap in the stroller.
Most people on the boards are going to tell you to go take the nap. We just dont get up and hit the parks by rope drop.. we get there by 10.. and stay all day. There are baby care centers when you can do feed the baby. They have changing tables, high chairs, a store to buy tons of baby items, a microwave and polar water. We just NEVER had a need to waste time going back to the room.
My advice.. try NOT going back the first day or 2. If they kids are tired and cranky and wont sleep in the strollers and are miserable by lunch time, then you might have to go back. I would make sure you have a reclining umbrella stroller for the 4 year old. It makes them sleep better !
Another thing we always do for down time with out leaving.. Make the hour before the parde, "down time". At MK. The parade is at 3. We head over to Main Street about 1:45. We get our prime parade viewing spot ( there is shade over there too) on Main street along the curb. We always sit by Casey's Corner. We get lunch at Caseys ( mmmmm hotdogs and cheese fries) and sit on the curb and eat. The ice cream parlor is right across the street, so we always have to get ice cream too. This accomplishes a few things.. 1. Great parade spot 2. Eating lunch 3. Kids are sitting down for almost 2 hours 4. ice cream!!

We STILL do this as our MK tradition!!