This allowance is for the main carer of a child or young person under 18 years who has a serious illness or disability and needs constant care and attention. Usually the allowance stops when the child turns 18 years (unless the young person starts to receive the Invalid's Benefit). Children can apply for the Invalid's Benefit when they turn 16 years.
There is no income or asset test for the Child Disability Allowance. A child may be able to get both the Child Disability Allowance and the Disability Allowance.
Talk to Work and Income for more information.