I have complained to retailers before about selling unsuitably sized tanks as "goldfish learner kits". As you have discovered, your tank is way too small for them.
If your fish have ich (and most likely) it will look like they have been sprinkled with salt. Fungus is fluffier.
Read the instructions on the Wonder Tonic and follow then carefully.
I suggest you see if you can re-home the fish as soon as possible (you will have to get them well first) then get something more suitable. Explain to your kids that a small tank is not good for goldfish and you need to give them a better home, then replace them with something a lot smaller.
Heaters are not expensive these days so it would not be difficult to go tropical and then your choice of fish would be wider. The only cold water fishes suitable for a tank that size are white cloud mountain minnows (already suggested) or leopardfish.
If you went tropical, you could have a Siamese fighter or a few cherry barbs, lamb chop rasboras, neons, micro rasboras, or danios. They poo a lot less than goldfish too!