That sounds awful! I'm sorry you lost chicks like that! Hard to tell what happened really, as many chickens will consume deceased flock members given the opportunity, but usually only if they see blood. If the chicken just falls down dead, most of the time I've seen them be left alone by others. But if there was something interesting or bloody, then they may have registered as food.
For hens that aren't laying anymore that you can't bring yourself to dispatch - I'd post on Craigslist Farm section - it allows sales of livestock. Just state that you have some 3 yr old hens who may or may not be laying - either they'll get eggs or a hen for the pot. Price them according to what you see on the site, and drop the price until someone takes them. If you post them for $20, you may have some takers. Unwanted roos for soup usually go for $5 or so, so somewhere in between those two, depending on the area where you live might be a good price.
You can raise up chicks without worry as long as you keep them separated until they're old enough to run away. Provide plenty of cover and places the older / larger hens can't get to, hidden food and water, and by the time they can't hide there anymore, they'll be old enough to defend themselves from the older hens. I'd be surprised if you had the chick eating problem again.
Could it be? Are they already done? 
