Without regard to this particular policy, those are all foods that your young relative is better off avoiding along with sesame seeds. I have been forced to avoid them for many years now due to different health issues, and it turned out to be a blessing that was hard to appreciate at the beginning.
Do you have some issue with carbs? High calorie foods? Something against processed foods? Gluten? Plastic sensitivity? Why was it a blessing? What made you start appreciating it? etc. etc.
I wish I had started younger, but nothing short of that kind of compulsion would have made me. Few kids need to go to such extremes, but most would benefit from the absence of flours.
Now that I think of it I wonder how much of 'gluten issues' is actually due to all those foods having excess sugar added to it
IMO flours have been damaging people in many of these ways for centuries. Since the agricultural revolution. That damage came along with a nice perk -- not starving. But if now we're affluent enough to not starve without it, that's a good thing.