Perhaps I should have said "blithely". We are also talking about a boy who did not yet understand the depths of grief, a child . . . yes, but a true "innocent". The same innocent who lobbied hard and didn't really understand the reasons why he was told he could not accompany his father to France to bring his mother home. That is why I believe he said: "a child should never be asked to do that".
To put it simply, he did not understand at all what he was going to do except walk with his dad who also didn't know what to expect. Harry has spent the best part of twenty years trying to reconcile himself to the consequences. Tony Blair et al did not know if William would accompany them until the day of the funeral.
Prince Philip had not planned to walk but he said he would (I'm guessing) to support William whom (I believe) would not leave Harry to walk without him. That this was a unique occasion is emphasised by their joining the rest of the family to walk behind the Queen Mother's gun carriage.