Are *you* going to try and get a bill through Parliament to make it illegal for under-18s to ride bicycles?
And £10 is too much. Vehicle tax (there's no such thing as road tax) is generally based on the amount of wear a vehicle places on the road, which is generally propotional to the cube of the vehicle weight. That means a bicycle licence would be something in the region of 2x10^-4 pence.
Alternatively, as vehicle tax is moving towards, carbon dioxide emissions - so a bicycle licence would be in the region of 6x10^-8 pence.