I find that smaller bichirs will eat pretty much the same things that larger ones will but in smaller portions. Appropriately-sized pieces of frozen 'krill', beef heart, clam, etc. are good. Frozen bloodworms are often eagerly taken by young bichirs as well. I have also had pretty good success with broken up Hikari Sinking Carnivore Pellets or other 'meaty' prepared foods.
-Joe