Is she coming home over night and then going back to be treated further, or has she been treated already and is now coming home, and the cage rest is part of the care instructions?
If the vet has already treated, then yes, sending her home with instructions to have her sleep in a crate/cage at night is normal. My cat broke her sternum, which can't have a cast placed on it, had to be kept quiet and crating at night was recommended as a way to keep her from moving around, injuring herself further. I suspect a pelvic fracture is also in an awkward place to put a cast and kittens are even more likely (full of energy and mischief as they are) to start moving around trying to play the minute they start to feel better and possibly seperating just-joined fractures.
Dr. Seuss~DLH (brother's cat)~June 2007-
Misty~DSH (my cat & Mooby's mom)-?-
MooBoots(Mooby) ~ DMH(Mom's cat)~July 21, 2008-
Sunshine~ Golden retriever X white german sheperd (Dad's dog)~October 24, 2008-