Oh no, you!

We had to put down our neighborhood conglomerate appropriately named Tuvix when my son was four. We didn’t do any of that paradise-farm-fairytale-BS. We were honest about why, and explained it to him.
Kids are resilient and adaptable, even if something sad happens. I find that honesty goes a long way.
He wasn’t present at the vet, we told him a after the fact (He knew that Tuvix was ill).
Personally I would shrug it off, as I know it’s temporary and said in anger. But I do agree with FryerDan that hilighting the nuance is probably a good idea.