I would take a child to a funeral at almost any age. Death is part of life.
The Royal children must be only too, too aware that their great grandmother has died. I am sure they have watched some of the pageantry on the television. It will have been talked of by their school friends.
If the family has a sensible attitude to death, and these things are talked about openly then there should be no problem.
When my DS was the same age as Prince George, a school friend and his brother were killed in a horrific road accident. It was on our library day. DS chose several books from the children's library that dealt with the body and an American one dealing with death. We went home read them together and talked about them and when, as was, inevitable, all the awful details of the accident, the ones that the papers didn't publish, made the rounds at school, he was able to deal with them with equinimity.
Look how primary schools deal with the death of a pupil, especially when in tragic circumstances. Sometimes some children go to the funeral or watch the procession.
Prince George has caring and thoughtful parents. Whatever their decision he will be OK.