When using either character, simply stay on the yellow tiles, and only use the crumbling tiles in an emergency. When you use them, stay on them for a while and jump at the last minute. You will be able to get back into the tunnel. This level is rather easy and you should be able to pass it with no problem.
Or, if you have the Child, you could step onto the crumbling tiles, not touch anything, and watch what happens. Enjoy!