It's irrelevant that many of us have children that attended a funeral at 9 years old (or younger) and weren't unduly traumatised, and behaved in an appropriate manner.
This isn't about whether a 9 year old should or shouldn't go to "a" funeral, its about a funeral that will have thousands of guests, tens, if not hundreds of thousands of people lining the route, TV cameras and photographers homing in on individuals, and because of this, they may well feel unable to express the normal range of emotions that one might feel at a funeral.
We had MILS funeral during covid restrictions. The service and wake was family only because of the limit if 15. Grandchildren ranged from 11 (my DS) to late 20s. All of them were in tears at various points and had a hug from their parents/grandad and probably half (grown up children and gc) fell asleep afterwards! Point being it can be an emotionally tiring experience and in normal family settings its quite OK to show emotion/have hugs etc....where as the RF are "on duty" and on camera so will be more guarded about how they behave (which in itself is probably more exhausting!)