In Super Mario Maker what kills Super Mario instantly but not normal Mario?

In Super Mario Maker (Wii U specifically) what kills Super Mario instantly but without turning him into normal (little) Mario?

My son wants to make a level where becoming Super Mario is bad and you have to go back and get hit by a baddie to turn small again. He wants a one-block-high tunnel to go through that only little normal Mario can enter.

He doesn't want to just put spikes in the ceiling, because it's too easy for Super Mario to just turn small again. So we want some obstacle that'll kill Super Mario instantly.

Solution 1:

The only way I see this as being possible is by making a one block high tunnel with the floor being fall-away platforms into lava.

Small Mario will be able to run over the blocks, but Super Mario will have to slide in, lose his inertia, and fall with the blocks into the lava.

Solution 2:

Rather than killing Super Mario, another option we found, to only allow little Mario through, is to have a small downwards drop first before the tunnel.

Now Super Mario can't slide in.