For newly hatched bettas (up to 1 week) you can feed: liquifry, microworms, vinegar eels, egg yolk, and infusoria.
For bettas over 1 week you can alternate: BBS, microworms, and vinegar eels. Sometimes I would feed bigger chunks of egg yolk, but its very messy so you have to vaccum the tank before and after feedings.
For juvies I fed mainly BBS and introduced crushed flakes/pellets/freeze dried BBS and Bloodworms.
This is just my experience of feeding baby bettas. It is imperative to feed them an excellent diet and keep their water extremely clean so they will grow at a good rate. And while bettas may scavange for food, babies will not be able to eat dead ghost shrimp because they only are attracted to moving/floating objects for food, and may not be strong enough to pick off pieces as TheEryieTrainer said.
Alexa