To me, what is ironic about this thought on the part of the father is that he is worried for the kids' sake when he ought to actually be worried for his own sake.
When George thinks that the kids are too young for death thoughts, he is worrying about how they will handle it. Will this stuff about death scare them? He is not sure they are ready for it.
But what he should really be worried about is his own life. Whether his kids are ready for it or not, the death thoughts they are having are about him and his wife. The death of some pretend zebras is not going to scare them -- they are planning for the actual death of their parents.
That, to me, is ironic.