The concern I would have, other than that dichlorvos is nasty stuff, is that it could be damping off disease that is killing the seedlings rather than the fungus gnats. Both damping off and fungus gnats thrive when plants are kept too damp, and enclosing the seedlings in plastic bags will increase the dampness. So if they are dying because of damping off then it would likely make it worse.
Since fungus gnats do their damage below the soil surface as larvae I don't know if the dichlorvos will reach them there as opposed to the flying adults, have you found some information on that?
