Well, first off, you shouldn't have bought it in the first place. Do you have an extra tank- at least 2G? Adjustable heater? Baby bettas are a little harder to care for than older ones. They need a warmer temperature, and higher-protein foods.
Get a tank, adjust the heater around to 80F-84F. That way, they'll grow faster. The babies in stores have been stunted in their growth, and might take time. After you got the temperature ready, you must be able to feed them frozen or live foods if you can. Frozen BBS are best. (baby brine shrimp)