like I said I have no experience with these but with my platy fry what i did was take the adult's regular flake food and just crushed it up super super small into almost a powder and just give him a bit. Or there's also this liquid fry food but I used the super crushed flake. I think it's just easier and more cost efficient since you probably already have the flake.
I don't think isolating the baby would be a good idea as they are mouth brooders so they raise the young in their mouth. If you have stock that could eat the baby I'd more separate the cardinal in general if your worried. I know also once the baby gets to the right size the cardinal will be looking for a spiny urchin or anemone to release the baby's to safely