I am nowhere near Bury but my boy who is turning 8 in May has been racing for 3 years so I'll ad my 2 cents. At first the 10th cars worried me a bit in terms of his safety so we switched to 1/18th scale cars and that is what we are doing now mostly. They are still bloody fast but weigh half of a 10th buggy and much safer for kids and adults.
We both have new 10th cars and he has been doing a bit of marshalling at Ardent and a couple of other places (and has had a full on hit on the ankle as well!) but he still likes doing it and is getting better at it. Better than some lazy arse adults I have seen.
I feel it is important to let kids marshal. They need to learn that it's part of the game. I always make very sure he is at the safest possible marshal point and have never had an adult refuse to swap with him. It is also important for the dads to take responsibility and stand there and marshal with them as appropriate.
I think a kids only heat does work in terms of racing. Whatever the kids age doesn't matter, you will soon see he is ready for the faster heats. We have had those at Wycombe indoors and it worked very well but you still have them marshalling the next heat.