1) You transplanted into too fine a soil. As they progress, they'll need something with a larger particle size, (regular potting soil) just to remain standing.
2) Ease-up on overwatering. They still need to dry a bit between waterings, but when you do water...you have to get it all the way to the lower roots, too.
3) Still a tad young for superthrive or nutes. Would stop now before you kill them. Give 'em a couple more weeks of straight (ph'd) water till they show signs of recovery. At most, a 1 or 2 internode seedling in seedling mix, will only need 1 drop of superthrive:gallon, once a week. Straight water in between. Around/after the third or fourth internode, can start thinking about nutes and additives.

If problems persist, post in the plant problems forum, and fill-out the troubleshooting guide. We can likely offer help much quicker, if we know the how's and what's of your grow.