I have something similar to what Mary posted. One way to prevent them from jumping over the top is never ever pick them up when they are in the ex-pen. Always open the door and pick them up that way, or let them out. Do the same when putting them in. I know it sounds tedious, but that way they associate the door with the proper way of entering and exiting the ex-pen.
If they figure out that jumping is a way to get out, well then kudos to you for having such smart boys! My boy is more than capable of getting out himself, but he never even tried. Good luck!
