As soon as they are free swimming they will be big enough to eat baby brine shrimp.Flakes are good too but nothing gets them growing fast like starting them on bbs
They won't eat until they are free swimming, because they are using nutrients from their yolk sacs, so feeding until then is a waste, and can degrade water quality.
I use frozen peas that I thaw and squeeze the 2 middle halves out of the shell and puree. It has worked well for many of my cichlids.