ok. What i would try to do - if down arrow is pressed - create object - CRAWLBOX (insert a box that is close to the size of the crawl space)
create that box so that it is in the same position as the player (make it 0% opacity) pin it to the player. Just like in the platformer tutorial.
Then if that crawlbox is touching the floor and ceiling objects - play animation - crawl.
Just off the top of my head. That's what i would probably try to start with.