A declaration by the person responsible for the child's welfare (e.g. a parent or doctor) is acceptable. This should be explained in Section 8 of the paper application, i.e. that the applicant is disabled and that he/she as parent/doctor has signed on applicant's behalf.
24-hour Passport Adviceline: 0300 222 0000 (see terms and conditions). Calls made to 0300 numbers from landlines and mobile phones are charged to customers at their network provider's national rate. Live operators are available to assist with your enquiry 24 hours a day.