J.T.
I nipped that issue in the bud when we first changed beds. I put my son to bed in his toddler bed. While in the kitchen I heard him walk up behind me. I never looked at him and told him in a very stern voice" You better get back in that bed right now...and do NOT get out". I closed my eyes and hoped it worked. Turned around and he was gone. Back in bed. Perhaps not the kindest way to handle it but it kept him there for a year or more...now its a entire other issue of pushing the limits!