Some just say alfalfa should be just a treat once the chin is over a year, here for example, Chinchilla Rescue | Forever Feisty Chinchilla Rescue | Care and Health | Hay
which I would think would imply kits should be able to eat it as more then just a treat. Some actually say to feed chins timothy hay and alfalfa here for example Chinchilla Food, Treats, Diet & Nutrition
Here are some examples of sites specifically suggesting feeding alfalfa to kits.
There are many different kinds of hay, but which types are best for your chinchilla? Here, we’ll share everything you need to know about chinchilla hay.
If your chinchilla may be pregnant, you must ensure you have a baby safe cage for when the kits arrive, as well as making the preparations to step in and hand feed if needed.
Not sure what to feed your chinchilla? This article goes in depth on the best nutrition for optimal results - written with the members of Let's Love Chinchilla's. Pellets A Chinchilla's diet should be simple. A quality pellet, ample amounts of fresh hay, and unlimited access to water. If...
Also recommended on FB pages here is an example
As well as the feeding suggestions on bags of alfalfa themselves like Oxbow's Oxbow Alfalfa Hay
I'm sure I've seen it suggested on Youtube videos as well, but I don't really feel like searching those right now. I'm pretty sure at least one of the old books on chinchillas I have says to feed alfalfa, though I realize a lot of the info in those books is outdated (but still recommends it there regardless of when it was printed). I've even read on this forum in the past people saying they know breeders that only
feed alfalfa and alfalfa hay/hay cubes no timothy and some claiming that breeders don't give chins any hay at all. There is a lot of varying opinions out there unfortunately making it confusing.
I realize these are not all creditable sources but showing some examples of how commonly it's recommended, I don't know, maybe all those places are wrong and kits shouldn't have any alfalfa.
It makes sense to me that a growing chin would need more nutrients in their diet then an adult, but I'm not a breeder and only dealt with a few kits over the years.