There is a separate award for the Young British Life Photographer. Young people can enter images that fit into any of the categories.
Those under 19 years old on December 9th 2016 are eligible to enter the Young British Life Photographer category.
Photographers who are eligible for the Young British Life Photographer category are not eligible for the adult categories. You can make an account for your child and enter on their behalf, just make sure you say in the caption that they are the photographer, and give their age. Note that to make any new account you must use an email address that has not already been registered.