Umm, aluminum? Stainless? Maybe even Sch. 80 PVC. However you do it, it ain't gonna be cheap. Unless you're doing it for the sheer novelty of it, or you have access to a lot of scrap stuff you can get for free, you're going to be close to the cost of a Flybar - a "professional, performance" pogo stick, if such a thing could be considered professional. The K&J website shows one ring magnet floating above another with approximately 4.25" separation. Just as a gut feeling, that doesn't really look like enough "spring travel" for a pogo stick, especially if you add just the static load of a rider. I'd say you're probably looking at max 3" of travel once your (young? small?) son gets his weight on it. Again going on gut feeling, I would think you'd want at least 9" of travel available. That takes four of these magnets, at a cost of $25 each. Then you've got the shaft the magnets would have to ride on. Unless you use Sch. 80 PVC, your material cost for the shaft alone will be close to $50. Then, as has already been pointed out, you have the fact that the relationship between force and displacement for magnetism is not linear like it is with a spring. Theoretically it follows an inverse square relationship, although in practice the actual relationship is nearly incalculable.
Another issue is debris. For the pogo stick to work, you're going to have to have some sliding fits, etc. in your mechanism. Unless you use it inside in a clean gym or warehouse, magnets that strong will pick up all manner of dirt, gravel, etc that has any iron content at all. It'll gum up the works and be nearly impossible to clean.
On the upside, I really don't see any issues/difficulties with assembly, as long as you're careful not to get the magnets too close to each other or any other ferrous item during pre-assembly handling. Once they're slid onto the shaft opposing one another, they can't spin around to attract and slam together. You would want to put them on last, though, since working around them with steel tools could literally be a pain if you're not super-careful.
