for a hatchling i would feed it rosy fry or guppy/guppy fry.
or a cheaper alternative would be to buy raw seafood by
the pound and cut it into bite size strips for it. you can also
feed it live worms like night crawlers & meal worms and
chopp it to bite size.
edit: its also a good idea to dust foods with vitamins to
insure a healthy shell. i like to dust worms and feed
my turtles out side of the water in a separate container
without water.
Hatchling alligator snappers are typically in the 4.0 to 5.0 cm range. So they are large enough to take half grown or larger guppies, Gambusia, and the like.
Until they are about a year old, I've had ZERO success getting them to take anything other than live minnows. It seems they are pre-programmed to lure for small live fish.
Some other keepers may have had success getting them to dead foods but since I find it easier to simply provide the feeder minnows, I've stuck with that approach.