Back in the days when we were allowed families days in the RAF and were able to take wives and children flying in the back of helicopters, the same rules applied in that you couldn't fly the aircraft your family were in so I understand where HMTQ is coming from regarding Wills, Kate and the kids.
But that rule also applied when had our UAS families day. Pilots could not fly their own wives in the SEP fixed wing aircraft (Bulldogs back then).