I would recommend a sponge filter, they are cheap work well and keep dissolved oxygen in the water, also won't hurt fry (unlike many other filtration methods)
You could try to culture infusorians, just put a jar with a little chopped up vegetables (like lettuce or a bannana peel) keep it in a will lit spot, but not something super sunny. Once the water gets cloudy you can just siphon it off into the tank, keeping some so you can add some more water and thus keep the culture alive.
Another thing to try is taking some green water and adding some pond debris, and letting it sit for a while in a contianer, you will often get little creatures that eat the green water that you can feed to fish fry.
Don't expect all the fry to make it (unless you are a really experienced fish breeder you won't get survival rates to adult hood above around 70% of the fry).
I think hikari first bites would work, but just be careful never try to pour it into the tank as it is pretty unpredictable and every time i have tried it, I have super overfed.
If you move them to a larger container it may be harder to feed them, but it also keeps the water parameters more stable. In a fairly large container some fish will make it to a fairly respectable size even if you only removed the parents, and never fed them. However you probably would only have 5% of them live.
Hope these tips help, I am not super experienced with goldfish breeding, but I have read some about it, and have tried it, mostly I just let nature run its course though and in my 1000 gallon pond occasional fish will make it to adulthood (unfortunately it isn't the pretty ones). Goldfish were the first fish I tried to breed, but I was just a kid then and wasn't very successful.
If I were you I might put some daphnia in the larger tub, and then move the goldfish fry their later, (if you don't have any daphnia culture you can get them at most at most aquarium clubs, and many of the non-chain pet stores if you ask them).
Congrats on the goldfish hatching, and good luck.