Disability Living Allowance (DLA) is the main benefit for disabled children and is there to help meet any extra costs of being disabled. Any ill or disabled child may be able to qualify, even those who don’t have a diagnosis.
The charity Contact have produced a guide to claiming DLA for children. Download it here (PDF 691KB).
The best way to apply is to call the DLA helpline (0800 121 4600) and ask them to send you a DLA1A Child form. This is because they'll send you a date-stamped form, which will ensure the benefit is paid from the earliest possible point.
If, at the end if the process, you disagree with the decision, you can ask the DWP to look at it again. This is known as a Mandatory Reconsideration. AdviceNow have a Mandatory Reconsideration Request Letter Tool that will help.
If you are unhappy with the new decision, you can appeal.