To protect youth riders from overuse injuries and ensure all racing is held on an equal basis (if you ignore the pro level bikes and ), gear restrictions are in use in all British Cycling events.
It is good practice to for riders to ride with these gear restrictions throughout the year so that the appropriate leg speed and cadence is developed.
The gear restrictions are measured by the distance the bike will travel through one complete crank turn with the gears in the highest position.
British Cycling gear restrictions information.
This can be quite complicated so below are some gearing suggestions to try based on full size wheels;
U8 – 5.1m : 34 x 15
U10 – 5.4m : 34 x 14
U12 – 6.05m : 34 x 13
U14 – 6.45m : 39 x 13 or 42 x 14
U16 – 6.93m : 39 x 12 or 42 x 13 or 52 x 16
Try the bike gear calculator app
You will probably need to lock out either the front chain ring or some of the rear sprockets by adjusting the limit screws on the gear mechs (derailleurs).
Or you can buy a "youth cassette" that has more teeth on the smallest rear sprocket, resulting in a lower gear.