Different turtles grow at different rates and eat different amounts, so it is hard to give any definite rules for feeding.
However, I think you may not be giving them enough, my 9 mo. old hatchlings eat much more.
The best advice I have heard is to feed them in a separate tank, and give them as much as they can eat in about 20 minutes or so. I usually wait 10-15 minutes more for them to defecate, then back to the regular tank.
Have you tried any foods other than Repto-Min? My little guys love earthworms, meal worms, and feeder fish. I am trying to get them interested in greens, berries, and fruits too. Some people say it is best to get them used to a variety of foods while they are still young.
Good luck with the little ones!