1) Yes, as someone else mentioned, hiding is natural for the younger turtles. When they get bigger/older they will be more visible.
2) Your babies might be eating, but just not when you're watching. They will go after the critters [like meal worms] that get away and burrow. The first couple of years that I over-wintered my babies indoors, I kept their habitat stocked with pill bugs.

In addition to the advice everyone else has given, you might try getting a [postal] scale and weighing them periodically to be sure they are gaining weight.