This is the 31st level in the Main tunnel.
When using either character, simply stay on the yellow, 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 in the tunnel. This level is rather easy and you should be able to pass it no problem.
Or if you have the Child, you could step onto the crumbling tiles, and watch what happens.