So I think the key points I need to make clear to them in my answers are:
1. The magic circles only bind the devils to stay, not anyone else, so whoever enters them can walk right out again with no problem.
2. Powering the dam only gives you one negative level, so anyone over level 1 can do it and survive, though they'll need a Restoration spell afterwards if they miss the Fort save.
Here's what I've got so far, though I think these answers might be too straightforward, and I don't like the rhymes as much.
Magic circles only bindSummoned pit fiend and his kind.
Others who enter always find
They can leave as they mind.
Life is drained when lake is filled,Weakest peasants would be killed.
But stronger creatures, weakness feel,
Survive the process, and spells will heal.