Just pop along (if it's open) or email your local Sqn.
You'll need to be vetted for PVG (child Safety) but that takes no time and there's paper work to go to Wing / HQ and give you a service number.
I've been a CI for about 5 years now (after one of the kids joined and snitched on me as ex-RAF) and I've become qualified to run the range & carry out Weapon training.
It's very rewarding and keeps you out the pub.