I was thinking the same thing, but a regular trainer has a much longer range of motion than a child can take advantage of. His looked much more kid friendly. I'm going to relatives in a few weeks and they have an elliptical trainer - I'm going to try Rowan out on it, and it if looks helpful, modding one to get rid of the handles and building a tripod would seem pretty straightforward.
It also seems to me that the distance between the feet is rather large on the ellipticals I've seen, at least for a child's hip width. I assume this is due to the large flywheel or fan used for resistance. I think the one in the video had a pretty narrow gap between the feet, probably at least partly due to a simple, narrow flywheel, since his isn't for resistance training.
I immediately started thinking about how one could use 2x4's and plywood flywheels, along with some steel rods and clamps, to mock up an experimental elliptical machine for sizing. A more permanent version should be fairly easy to make with off-the-shelf components, and some welded steel stock.
