The child disability benefit (CDB) is a tax-free monthly payment made to families who care for a child under age 18 with a severe and prolonged impairment in physical or mental functions.

Eligibility
To get the CDB:
You must be eligible for the Canada child benefit (CCB)
Your child must be eligible for the disability tax credit (DTC)
If you are already getting the CCB for your child who is eligible for the DTC, you do not need to apply for the CDB. You will get it automatically.

Note:
A child is eligible for the DTC when a medical practitioner certifies, on Form T2201, Disability Tax Credit Certificate, that the child has a severe and prolonged impairment in physical or mental functions, and the Canada Revenue Agency (CRA) approves the form. You can send the form to the CRA at any time during the year.