My son is eleven years old and he has been competing with us adults in
Lajf Racing F1 Series this year it is a 8 races and one final serie so ther has been in total 9 races. Each race is 35 minutes. (web page in swedish language)
He had never driven a F1 beore this year. I got him a second hand F109 That was in need of repairs so I fixed it with F103 front end. The cars run 13.5T motors and fixed timing and a maximum gearing at 2.7. It gives us a very equal set of cars running, regardless of chassitype. Motor in my sons car is a sensorless 13T at 3000Kv ESC is an old mamaba max.
My son did OK, sure he's been struggling at times and he has probably caused a few accidents but then again which one of us has not? As the points stacked up over the summer my son ended up at overall third spot in the series.
I would also suggest that you go for the Tamiya chassie as they are very durable, choose the w version with the F103 front end as there are less stuff to brake in that.
Our kids need to be allowed to run with us to learn, sure they will be there right smack infront of you in your raceline but then you need to overtake by going around them. If they are supposed to be good drivers before running at your track, the hobby will slowly die.