size matters more than age. for some people they still look newly hatched even at week three, for others they look older than three weeks.
BBS and daphina should be fine. I'd wait until they're about half an inch or so to feed the cut up black worms, but you can pull out one worm, cut of an itty piece and see if any of your fry will eat it. sometimes they wait until it hits the floor of the tank to inspect it, sometimes they gobble it up right away.
Remember to clean your black worm's water/swish em around to get rid of dead ones and poop.