What's destroying my sunflower seedlings?
I just noticed this question was answered a year ago and the answer given was incorrect. The person asking the question pointed out that the seedlings weren't being eaten, but were broken off at the base of the stem. Whatever is doing it has no interest in eating any part of the plant. There is only a clean break at the base of the stem. This is happening in a garden that is fenced in and isn't accessible to anything larger than a chipmunk.
Cutworms will do this type of damage, they live in the soil. To protect your seedlings, put a 2 inch collar around them, made from paper tubes or yogurt cups. Cutworms collars are also sold by some garden vendors.