So sad :( have you tried posting in the predators & pests forum? Maybe post there and get suggestions? Photos of your coop/run so maybe others may spot a potential problem you could correct? I feel so bad for you. I know how much you care about them!
So sorry Juliet - that's awful! I hope the reinforcement does the trick, not just for the cats but any other critters that may try to get in. We have trapped a lot of raccoons, possums, bobcat... the list goes on. Chicks are irresistible to them all. The best "bait" we found was actually wet...