If they are total little pigs you can’t give them unlimited pellets, or they will get fat, but you can give them alfalfa hay in addition to their grassy hay. Alfalfa is high in protein and calcium and very good for growing bunnies. It’s also yummy. Mix it with the grassy hay, because you do want them to eat grassy hay as well, the sooner they start eating grass hay the more likely it is that they’ll become good hay-eaters as adults – and that’s the best thing for good bunny health, lots of hay.
How much pellets you should give depends on the size of the rabbits (weight). If you have kitchen scales you could weigh them weekly in a kitchen bowl and make nice little weight curves for them so you see that they are increasing in weight like they should. Then you adjust the amount of pellets accordingly.
You can also scatter their pellets around a bit so it takes them longer to find them and eat them. That’s good for another reason as well: very greedy buns sometimes happen to inhale their pellets into their airways, and that’s very dangerous.