You already know my vote so I won't bore you with it again but I can vouch that High Wycombe is a great and child friendly club and they run Saturday evenings. If you haven't had a chance to get round there to see it on a Sat eve then pop along as soon as possible and have a look.
Also, I will add that allowing my kids to play with their cars at home whenever they want has gone a long way to create the desire for them to want to do it again and again and not feel any apprehension when going to a club and taking part in the racing. My boy is 8 and he done nearly 3 seasons of BRCA micro nationals.