Question
You may have answered my question, already.  However, I was SO PROUD of my Brugmansia seedlings ... they reached 3" in height and then ... DIED.  I kept them moist, not wet.  BUT, perhaps I fertilized too often ... I used a very diluted general houseplant fertilizer ...  

Answer
Hi Jim,
Thanx for your question.  I would use some milled sphagnum moss around the brugmansia seedlings.  I put a layer of milled sphagnum moss on top of the soil after planting the seeds, about 1/8 inch in depth.  This really helps keep down the fungus that causes damping off disease to which brugmansia seedlings can be so susceptible.  I lost all of my B. sanguinea seeds on two occasions to this disease.  Also, use a small oscillating fan on low to keep the air from standing which also helps foster damping off disease.  I wouldn't start fertilizing too much until the plants were 6 or 8 inches tall.  You might try half-stength instead of full strength but I really think it was damping off disease that killed your seedlings.  I hope this helps.
Tom