Common misconception! You actually *don't* need to hold down, you just need to be crouched. Holding down will cause trouble as you'll tumble off platforms etc. We're currently looking into solutions for making this clearer.
You could require players to crawl through a low area in the beginning, then have them do a long jump while standing on a pipe directly after the short crawl so that they understand that the long jump can be triggered without holding down!