The problem in the UK is that a lot of MTB’ers do not know the correct trail etiquette!
Only on a black does a faster rider have priority. On green, blue and red, slower riders have the priority! However on a red, slower riders should be considerate to faster riders, (if the faster riders are being considerate too!) and let faster riders past at the earliest opportunity.
Yes youngsters should be allowed to ride whatever trail they are capable of, I know a lot of sprogs who can beast grown ups on some blacks, certainly if they have BMX experience, they are usually ace on the ‘jumpy’ trails. However, I would recommend that the parent/grown up acts as ‘tail gunner’ to protect them against the strava w4nkers.
At a lot of trail centres I often see grown ups walking past the first obstacle on black runs – if you can’t do that feature, you shouldn’t be riding that trail!!