Dog food is no good for a creature that is a herbivore, ie vegetarian. Just feed cabbage, lettuce and carrots. you can chop the carrots into matchsticks to make them easier to eat.
Rabbits like all the raw vegetables you can't get a 3 year old to eat cooked. They don't like their food cooked, though, so don't try to serve them the leftovers.
Grass is better than dog food, but they used to tell me when I was a kid that if you fed a rabbit entirely on grass it would explode. The wild ones I saw lived mostly on grass, and they kept on running around, so I knew that couldn't be right.