You can get HDPE for the same money as decent wood, and you can use wood tools to work it. It's significantly... well, better, for combat, than wood. A 15-20mm HDPE box will be more or less impenetrable. In the long run it's probably cheaper than wood. Plus, you can compete with the new guys or stick it in with NST and Inertia XL for a laugh. With wood, unless it's crazy thick, you just can't.

For that reason I don't think this class will ever be much more than a novelty. You can't turn your first robot into a full combat machine if it's wood, without literally rebuilding it. It isn't cost effective unless you stick to only competing against other wooden robots, and a 12 year old who is new to the sport will quickly get bored with that.