Without pix, hard to say.
Too early for extra nutes or additives...they can too easily burn. Especially if using MG.
How large are the pots they are in? How many in each pot?
Do they have adequate drainage?
Do you let the soil dry a bit between waterings?

The MG may be a bit hot for your babies, however I've never lost a seedling to MG. But I did have to learn that they are serious when stating on the label that shit about being a moisture control formula. Extra perlite, and a tad longer between waterings helped. 1/2 strength nutes (or so) after the first 3 or 4 weeks, regular strength a couple of weeks later.