Oh, Tricia . . . I'm so sorry to hear about your kits.
First thing to do is to figure out WHAT has access to the rabbit cages. I don't think mice would be an issue, but rats are certainly a strong possibility. Raccoons will also pull things through the wire and eat as they go, so if your cages are outdoors (sorry, can't remember) then they could be the culprit.
Assuming it is rats, you are not alone. Rats are terrible everywhere right now, it seems. Truckinguy had a major infestation recently. See his thread about it:
mothballs-for-rats-t32446.html
I've trapped 27 rats in my kitchen alone in the past 10 days. All juveniles, so that means the adults are still there somewhere. I won't use poison in the house because of the likelihood of decaying rat carcasses causing a stink. I use the heavy-duty black plastic pedal-style traps. Most of ours came from Canadian Tire but you can also get them online. TomCat is one brand and T-Rex is another, IIRC.
I think the plaster and icing sugar mix is safer to use than the one-bite baits that are available. There is no antidote for the new poisons in the baits, so if your barn cat were to eat some -- or even a poisoned rat -- it could well be fatal. Another advantage of the plaster and icing sugar mix is that because it is a powder, the rats have to eat it in the place it is offered; they can't carry away chunks of it that might be ingested by other animals. If you place the plaster mix in a container inside a covered bucket with the only access being rat-sized holes in the sides, it should keep it away from other critters. A cat would not find it enticing in any case.
My suggestion is that you buy some of the rat traps and also use the plaster and icing sugar. If you place the rat traps on top of the cages, chances are that your barn cat will be safe. Peanut butter mixed to a stiff paste with brown sugar is a fairly good bait. I've also had astounding success by using some leftover fish baked with butter, tomato and onion. The black traps have a nice little cup to hold the bait which makes it harder for the rat to get it without setting off the trap. Rats are wary, so a few tiny bits of the bait put in front of the trap will let them get a free sample and lower their natural caution.
I'm still exploring options to find the best baits. Rats are attracted to the smell of very ripe bananas/banana skins, and also to anything smelling of sulphur (hard-boiled egg yolk, for instance). I haven't tried the egg yet but I remember how enthusiastic the pet rats that MidnightCoder had as a kid were about hard-boiled egg.
Good luck. Even if you don't get them all, they seem to only bother kits when their population swells and food becomes scarce. If you put a dent in the population, your rabbits should be much safer . . . though you should keep after them anyway, or the problem will come back sooner or later.
The very BEST remedy is having a weasel or mink take up residence. They can be a problem in themselves, but they will eat a ton of rats first and can usually be easily caught with a live trap baited with sardines.
