Originally Posted by Cat Herder:
I have always presented the mat as a reward. 
"If you can lay down like a big kid you can stay on this side (half wall) WITH the big kids. If you get up or make noise, you will have to go back in the crib on the little kid side. ok?"
Then I follow through, every time. 
It doesn't usually take long.
***Now, the mats are not well viewed from the cribs on the infant side of the playroom so it is a social motivator.
That's a good idea. I did move him into a separate room, we call it "the training room" maybe I need to talk to him about it more and explain that he can stay in the "napping room" with his friends if he is quite. He is not super vocal, so I'm not sure how much he understands me.