I have three Silver Dollars. Out of them, two of them will only eat algae discs, the other one will eat algae discs and frozen brine shrimp.
I'd try buying some frozen or live brine shrimp and some algae discs.
Note, if you try to do algae discs you might want to get the algae thins depending on how small the Dollars are. My 3 inch Dollars eat algae thins, my full-grown Dollar eats algae discs. If they're really tiny though brine shrimp or crushed flake would probably be your best bet.