When you take out the family tariff, you have the option of registering an additional adult and up to 3 children as insured persons. By children we mean your children and the children of your spouse, registered partner or common-law partner if they are under the age of 21 (including foster and adopted children).
The basic requirement for insurance cover is that these people travel with you, since your A1 phone number is the trigger for insurance cover (“roaming”).