Annie
Try the cadet kit shop, used to be run by the Arm,y Cadet Force UK HQ at Frimley Park but i now believe its a private enterprise near Birmingham area
here is a link to their site for Ligtweight Assault Boots Size 2 - 14 But you will need to ring them. read the Description it states 2 - 14 but their size on line chart shows 3 - 13
Give them a ring they can be quite helpful.
Cadet Direct - GENUINE LIGHTWEIGHT ASSAULT BOOT, BRAND NEW
Also, ask the SCC OiC as they may have smaller boots in the detachment from younger cadets who have grown out of their boots quickly. Some run a boot exchange type scheme.