Harry wanted to walk behind his mother's coffin. William didn't want to. Prince Philip told William and Harry if they wanted to walk behind their mother's coffin he would walk with them and support them. I don't think they were forced to do anything.
With respect, I don't think that children of their age should have had the responsibility of making the decision. Why was it even an option? At 12, Harry did not know what was good for him, and the responsible adults should have known better.
If William had a choice now, of a 12 year old George walking behind his or his wife's coffin, it simply would not happen. Nor would Harry be happy about a 12 year old Archie in the same circumstances.
Hindsight is 20-20 vision and all that, but that decision, whoever made it, was 50 kinds of fucked up, it should never, ever had happened.
Harry was much too young, even if he made the choice himself. There is a reason children are not allowed to make certain decisions.