Hi, I'm very sorry to hear about the babies! Around this age young rabbits commonly have digestive problems, as the Ph of the stomach acid is changing, and the bacterical balance of the intestines is easily upset. At 6 weeks they should still have been with the mother, as although does commonly stop feeding around this time, the young rabbits eat her cecals to ingest the bacteria they need for a healthy digestive system. Often the only symptoms are the baby will still looking uncomfortable, perhaps grinding it's teeth in the later stages and then die a day or two later. Being fed the same food as they were weaned on it vital at this stage, a change of food at this age is the most common cause of this problem. I think this is the likely reason, as you said the baby was bony and suddenly going "thin" is usually a sign of severe dehyration.

Another possibility is a bacterial infection which can also be common in rabbits around this age. This has more symptoms though, and you would likely have noticed loose stools. Depending on the type, these can be spread so I'd suggest keeping any other rabbits away from the hutch of the sick rabbits. Keep a close eye on the rest of your buns, make sure they are eating/drinking and most importantly going to the toilet okay, not pooing is a real cause for concern!

It is always good to have a tiny syringe for rabbits, as if they get a digestive upset they just stop eating, which is the worst thing for them! If you have a bun who looks uncomfortable or in pain and refuses to eat you will need to force feed fluids to them, they need a lot of water, and vegitarian baby food is good to help keep them going. You must syringe feed on a flat surface, and sideways into the mouth to avoid squirting down the rabbits throat as this could flood the lungs. Infacol is a great help to rabbits in times like this. A rabbit in this condition should be fed at least every two hours and taken to a rabbit savvy vet as soon as possible.

I hope your other rabbits are okay.