You really do need to be giving a greater quantity of pellets due to the growing needs of the baby. When I fed my baby buns, to make sure they were getting enough nutrients from pellets as well as eating enough hay for good digestive function, I would feed enough pellets that it would last most of the day but that they would run out of pellets about 3 hours before their next feeding, and I fed twice a day 12 hours apart. So I fed enough pellets to last 9 hours. Then in those last 3 hours the bunnies would eat their timothy hay before I gave them pellets again.
If the pellets contain alfalfa and are a high protein pellet(16% or higher), then you really don't need to worry about providing too much alfalfa hay if you are making sure to feed enough pellets, and can mostly just feed the grass hay along with the pellets, though you do need to make sure your bun is eating the grass hay well enough in those few hours before giving pellets again. I would maybe only give a little bit of alfalfa as a treat.
If you haven't been feeding the baby bun very much pellets, then you shouldn't just suddenly increase the amount. It would be better to gradually increase the amount over several days to give the bunnies digestive system time to adjust to the change. While you are gradually increasing the amount of pellets, I would probably continue to feed a bit more alfalfa hay so the baby bun is getting enough protein, and just gradually decrease the amount of alfalfa hay as you are increasing the amount of pellets. And keep a close eye on the baby buns poop to make sure it doesn't get mushy. And always make sure the baby bunny doesn't run out of grass hay.
The reason that it is important for baby rabbits to get enough pellets is because of the extra nutrients they need for growing. Yes technically they could survive without pellets like wild rabbits do, but the problem with this when it comes to domestic rabbits is that wild rabbits can pick and chose which plants they eat so that they can ensure they are getting the needed nutrients, such as baby bunnies being able to eat early growth grass that is much higher in nutrients and protein than the typical hay that most domesticated rabbit eat.
Feeding a baby bunny a no or low pellet diet just isn't usually doable for most pet owners. Pellets are an easier way for us to make sure that baby rabbits in particular, are getting enough nutrients for growing. It is possible to feed a low or no pellet diet to domestic rabbits(I have one adult rabbit that can't have pellets), but it usually requires very careful planning, monitoring, and extra effort to make sure the rabbit is getting all the needed nutrients each day. If the pellets caused mushy poop or other poop irregularities, then that would be one reason to provide a low or no pellet diet to a baby bunny, but again it just has to be done carefully so the baby bun is still getting all the nutrients it needs for growth.