@ineedaholidaynow "I also have to say, that when it's my time to shuffle off this mortal coil, if I knew there was a 5 year old grandchild/great grandchild who had the choice of taking part in sports day or attending my funeral, I would want them to do sports day and possibly with a parent cheering them on. They can remember me in different ways"
This, in spades. I was close to all my GPs, unfortunately all now dead. But I can't imagine any of them wanting their great-grandson to have no one at sports day (or any event that's important when you're 5) so that DH could attend their funeral.
Every family has a different attitude to funerals. In some, it's seen as critically important that everyone should attend. In others, it's not. My family would be in the latter camp... in fact, funerals in our family have often turned into quite cheerful events, with lots of family and friends around.
OP, if you, DH, and whichever of his parents the grandfather "belonged" to are all happy with the plan, then I wouldn't give it a second thought.
You can talk to your DS about his GGF, and figure out an appropriate way for him to show his respects. At 5, it's perfectly reasonable for him to not understand and quite possibly to not be too bothered. It doesn't mean he lacks respect or will grow up to stab elderly people in the street.