For a little extra inquiry, I decided to use two sponges, one with antibacterial chemicals and one from Trader Joe's. Both work! Grass seed can be found at local nursery, but was only sold in detergent sized containers at the one warehouse store I checked. B's scoopful only cost $0.33 and there were wagons at the nursery!
First wet the sponge. The Trader Joe's sponge came compressed and watching the expansion was a hit. I gave B a spoon and dropper to extend the play, but this is not necessary.