I feed my hatchlings a mixture of commercial turtle food. I start feeding them Zoo-Med hatchling pellets a week after they hatch. After a couple of weeks I start offering them a variety of commercial pellets. Omega One and Nutrafin Max foodsticks are both excellent, I also offer them Mazuri turtle pellets and Aquamax.
I feed them once a day and only as much as they can eat in fifteen minutes. Once they reach 3 inches I suggest feeding them every other day.
In my opinion the most common mistake people make is feeding their hatchlings live food, fresh seafood or krill as treats. These foods are very appealing to diamondback terrapins and unfortunately they can quickly decide that they no longer want commercial food. It is very common for them to refuse to eat anything other these treat items. Once this happens it can be very hard, if not impossible, to get them back eating commercial foods.