Just as a head's up, Ben was about J's age when when we lost a cat. He acted similarly when we told him - "okay, that's sad, can I have a snack?" He's eight now and he still has random bouts of delayed grief. They seem to come out of nowhere, but we comfort him and look at pictures. Talk about what a great life the cat had and how it's okay to be sad. Let's make sure we appreciate our pets while they're with us, blah blah blah.
When FIL was sick with cancer, he had taken a turn. Dh and were so worried about how dd would handle seeing him. Turns out she did much better than we did.
I've learned several times over the past year that we don't give kids enough credit. Somehow they have it figured out better than we do.