Villagers are not affected by soul sand height.

Steps to replicate:

• Make a 1 wide x 2 tall hallway 8 blocks long w/ a ceiling.
• Use soul sand for first four floor blocks
• See that you (the player) are unable to walk from the soul sand to the regular floor
• Spawn a villager on the soul sand
• See that the villager can walk from soul sand to regular floor

What you expect to see – villager (adult or child) are unable to walk from soul sand to regular floor.