Have you tried making competative for the older kids? Like saying who can pick them up the fastest?
I know we always sing that clean-up song however it's suppose to go.
I would definately make some sort of consequence for not picking up at age 4 and older they can definately be responsible to help out.
I know when I worked at a home daycare we only had cleanup time twice a day. Otherwise we left the big mess. It went kinda like this....
Free play, left the mess to go eat breakfast, after breakfast they could go back to free play and we usually set up a craft or preschool lesson then they came over and did that again leaving the mess, after craft time they went straight outside snow, sunshine if it wasn't freezing or raining they went outside. Then they slowly trailed inside back to their free play while we got lunch ready. Then we'd call them over for lunch and while we were cleaning up lunch they sometimes had a little bit of free play or they knew it was clean up time and when things were clean they got a video or story and got in their calm down mood for nap time. So they had alot of little spurts of free play here and there and we didn' bother with the mess until after lunch so there was no nagging or hassle. After nap time they woke up and sat on the couch and watched a movie or something while we prepared snack time, if it was good weather they went back outside after snack until the parents arrived. If it was bad weather we got out a few toys or did a activity and maybe a 1/2 hour before close time we cleaned up again if needed and just read books or something that didn't make a mess. I guess by only worrying about clean up 1 or 2 times a day we didn't stress the kids out or ourselves. They had limits with the toys too, they could only stay in a certain area and she had a back room she kept the toys in adn she only pulled out a few bins a day to keep the mess down and to rotate toys so they wouldn't get bored.