Convicts are omnivores, or detrivores (AKA scavengers)and in nature they eat everything from insects, fallen leave litter and fallen fruit, seeds, algae, and other detritus.
I have fed fry everything from crushed thawed peas, to cooked egg yolk, to brine shrimp, mosquito larvae, and crushed pellets, and it has all worked.
I never did anything but feed the adults. The fry would eat the small particles from the food the adults ate. Im sure they also ate algae and other sources of food.
Hikari has a fry food called First Bites. As expected it's super fine so should turn off pumps while feeding. I used it with good success with my Amphilophus fry.
NLS Grow, used it for all my fry for many years. The fry in with adults mostly just got what the adults ate. Fish are so resilient any decent food will work, some just work better than others.