I think the very little one's should have some low-intesity UVA/UVB full spectrum light source (and a way to hide from the light) because they are growing fast. Calcium/mineral supplement powder on a worm, or whatever, now and then isn't a bad idea either, but don't over do it. Sounds like your little guys that are doing OK will make it just fine. Feed them often, but don't let them get over stuffed. At that age, occasionally one will just up and die though. Doesn't necessarily mean you're doing something wrong. You should stay away from red (tiger) worms though. Night crawlers should do nicely, even if you have to chop them up.