I think actually they were a good age. Old enough to understand enough, but young enough that joining in makes them feel grown up and the pomp and ceremony exciting rather than a bore.
When dd was 5yo her beloved great grandma died. She chose to come along to the funeral, and it was the right decision for her. I can say with confidence that if it had happened when ds was 5yo he would have chosen to stay behind, and it would have been the right decision for him.
William and Kate are parents, just like me, just like most of you. They are making decisions for their children, and it will make it far easier for them to make the right decision if they don't see this sort of thing picked over on the internet/press.
Let's trust them to make the right decisions for their children-and if sometimes they do make a wrong one in our opinion, well, I've no doubt all of us have sometimes made wrong decisions for our children but with the best of intent, so we're all in that same boat.