K.W.
Do you need them to stop sleeping in the crib? The reason I ask is that I tried the same thing when my son was 2 1/2 and had the exact same problem--nighttime sleep fine, but no nap at all, and he though it was so funny to get out of his bed at nap time. By the 3rd day of not taking a nap--I was scared he would drop his nap completely. When I brought this up to my other mom friends they said "does he sleep in the crib fine?" "yes." "Does he try to get out of the crib?" "No" "Then why are you trying to make him sleep in a bed if you don't have to?"
So I put his crib back up and he was fine.
We got him a big boy bed at 3, put it in his room for a couple of months and then transitioned him to that without any problems.
So if they are sleeping in their cribs fine and you don't have another one on the way needed the crib--just keep them there--you may be just causing problems where you don't need to.