Child Tax Credit has been replaced by Universal Credit for most people.
For those not in a Universal Credit area, Child Tax Credit is payable if you are responsible for a child under 16 (or under 20 if the child is in approved education or training). Child Tax Credit is means-tested.
If you are still on child tax credit and your child starts to get Disability Living Allowance (DLA), you may be entitled to more Child Tax Credit. Tell HMRC that your child is now on DLA.
You can only make a new claim for Child Tax Credit if you get the severe disability premium or got it in the past month and are still eligible for it.
Go to Gov.UK for information on Child Tax Credit.