Having just 'finished' a similar experiment, I hope I can offer some advice.
We've been owned by Black Java chickens now for just about 2yrs. I began to prepare our sons (at the time 11 and 9) that we were going to expand our 'farmette' to include Black Javas and why I chose this specific breed, etc. With some screening, and preparation on my part, I found a not-too-awful youtube video about conditions at a typical 'feedlot farm' (I think it was a turkey farm). I know Dirty Job's tv show did a similar one on turkeys as well as pigs - might go that route instead for your youngest daughter (Dirty Jobs is available on Netflix). Having had previous discussions about puppy mills, my sons were fairly familiar with the set-up and rationale behind why our family does not agree with this type of 'farm' and how we can change our eating habits to reduce our family's dependence on things like Tyson or Hormel.
We broached the subject matter-of-factly - that yes, God makes all thing in pairs (even bugs!), both male and female - and that for our situation and purposes, the egg layers can only be girls and we only need 1 very special boy. I included them in the decision about which boy to keep (remember, they all had names at this point, had lived with us for about 6wks, each son had a 'favorite' and argued about who to keep!), but they understood that as primary keeper, Dad and I had the final vote. We ended up with 7 boys and 5 girls. We obviously kept all the girls. Two boys were out because they were white Java's - George and Little Bo Peep. 2 boys were out because they constantly fought with everyone over anything - Wacko and Star. That left 3 boys to decide between. So, I put up a 25' section of fencing and a dogcrate 'coop' and plopped the 'contestants' in this secondary pen to live in for a very short while while we watched their temperments.
I will tell you that one son was devastated to learn his first choice was not going to stay. He got over it once he discovered the cockeral who was staying was friendly if given mealworms from hands, and is now in charge of the mealworm farm to give treats to the flock everyday. He loves to sit with the chickens gathered around him waiting for their next tasty tidbit! The other son was and still is pretty ambivalent over the whole thing.
As for the processing part. As a Christian family who's church has a fellowship meal after service every Sunday, we proudly brought 'Chipmunk' to lunch as a first-fruit offering when he got big enough to be worth a meal. Obviously, Chipmunk was a guest of honor by being part of the main course! Sons were very proud to share what they helped 'grow' (yeah, I did the work, but in their mind visiting the chicks daily to pet them counted as 'care'!) - much as the pumpkin pie we'll bring in the fall again, or the apple cobbler or the spaghetti sauce lasagna - it's just part of the circle of life. Now, when we're eating chicken, the boys ask "is this ours?" and if I say no....they're not as anxious to dig into the meal! They too have figured out that homegrown tastes so much better, whether it is tomato or cucumber or chicken! Perhaps your older daughter will appreciate that fact.
Our now 13yo did help process some of our cockerals this past spring. Much like he helps process our catch of bluegill every year. Each year, he does more and more - and each year we watch him mature. Our now 11yo will begin learning in earnest this year to process bluegill (did try last year, but got startled when the fish jumped at the pressure of the knife). Didn't push the 11yo to come out to see the chicken processing. He will when he's ready.
Hope this long explaination gives you some insights. It's tough, but part of parenting. Your 3yo is old enough to understand some of this - that everything eats something else to live (chicks eat bugs, right?), and part of living is eating tasty things. As my husband says, 'things that eat tasty things taste tasty to me!" Good luck!
