Your tank looks fine for a small turtle. I notice that the UVB is in a standard clamp lamp.... is it a fold flourescent? Also, the UVB is rather high up. Usually they're only good for a few inches (check the package for recomended max distance). You've certainly got enough plants! I have fake plants for my eastern painted and she loves to hide in them.

As to why the hatchling that died.... without a necropsy, there's no way to tell. It's possible that the shrimp had parasites that were passed onto the turtle or it could have been coincidence. Hatchlings do tend to be delicate.