I feel you on this. My little girl is three and in addition to losing her great-uncle and my parents little dog dying (we live on the same property as them and she spends a lot of time at their house) we also lost 25 chickens and 2 ducks in about a month to a fox (most of the chickens were killed while we were away and a relative was watching them.) She's definitely learned more about death than I hoped she would have at this age. Like you, our birds were killed right in front of our house, and in the middle of the day
My dad and cousin who was watching our animals, have spent a TON of time and energy trying to shoot, trap, snare etc the fox. After returning from our vacation I invested in some electric poultry netting and haven't lost any more birds. If a predator has gotten a meal somewhere, it will always keep returning. In the two months I've had our birds behind electric fence I've seen the fox close to them several times (it always manages to slip off before I grab the gun! Just because I don't want to spend a lot of time and energy hunting it doesn't mean I wouldn't kill it if I had the chance.) The bobcat won't ever stop coming round. It will come and check, and if there's not an easy meal it will keep going and if it has an opportunity it will take another bird. Your best bet is to protect them with a strong psychological deterrent (a HOT fence).
Anyway, we deal with this with our daughter in a few ways. Firstly, we do not name our farm animals or make pets of them. Our sow who had piglets we call mama pig, and that's as close to a name as any of them have. We also talk to her (nature shows like planet earth help with this!) about how predators have to eat other animals. They don't have any choice and if they didn't kill animals they would die. Having said all that, she did have one buff orpington she named Pingus. When she says she's sad about the fox eating our chickens (which she does mention) I agree it's sad, but there have been a few times when she's said "mama, did the fox eat Pingus?" And I've said "I don't know sweetheart, maybe Pingus went to live in the woods.". Not my proudest moment, but I totally understand what you're saying about it being a lot to process for little ones (bear in mind, we left for a vacation with ~25 chickens, and came back to 6).
BUT kids are resilient! She talks about how she's sad Maddie (my dad's dog) or Uncle Mike died, and I try to always listen to her feelings and agree with her that it's sad, and make sure she knows if she ever wants to talk about it or ask any questions I am here, and she is processing and coming to terms with it in her own time. And really, I think one of the wonderful things about living on a farm is that our children get to see this real stuff of life. Chicks hatching, piglets getting born, and likewise pigs getting sent to the processor to be turned into meat, and death from predators and illness. I wouldnt want to raise her any other way and I don't think your are traumatizing your son. It just takes them a while to process death, the same as it does for all of us.