Wild baby rabbits would just eat the same plants and grasses as the adults do.
Alfalfa is available to us if we want to feed it. It is higher in protein than standard grass hay, and can be useful to help with weight gain and growth if it's an issue - but it's not essential. I would encourage good general hay eating habits as early as possible - it promotes good gut and dental health, which gives them the best start for the future.
Don't forget vaccinations and neutering as well.