Hi Jane,
Congratulations on your Kits!
But, of course, I am saddened to hear of your loss.
Breeding chinchillas can come with good and bad. You have just experienced both.
In my opinion, the loss of your kits lip resulted in the young ones fighting for food. In the past I have had kits that fight and the first areas they go for are the mouth and nose. With an injured mouth and nose they will not be able to nurse and die. It is a terrible thing to have happen, but it is survival of the fittest.
Best of luck with your new ones....keep a close eye on them. If you see the kits fight, you may have to move one to a surrogate mom or take turns giving the kits one on one time with mom so they can get individual nursing time.