Graham,
My son is 6 and is very keen too!
I live in St Leonards, nothing around here, so i contacted a chap at Lydd who was very helpful, but they are awaiting a delivery of minimoto bikes, he said to call again in a couple of weeks.
I then remembered Brands Hatch, there is a company at the back near the bar called Adrenalinx (I think thats the correct spelling!) I spoke to a very helpful girl there, I think they do try out nights on Tuesday and Thursday from 5 o/c to 8 o/c,
they provide a bike, helmet gloves and protection all in I think it's about £55.00 not bad eh.
My son is going to Greece next week with his mum for a break, so he is going when he gets back, as mummy didn't fancy taking him on a plane with a broken arm or leg
Hope the above is of some help, please get back to me and let me know how you got on
cheers, Neil