If you receive Working Tax Credit you may also be able to receive Child Tax Credit for any 'children you are responsible for', as deemed by certain criteria listed by .gov.uk. You can make a new claim, or make alterations to an existing claim due to a change in circumstances, etc. The amount that you receive depends on the number of children claimed for and their respective dates of birth. To be eligible, the child you’re responsible for will need to be either under 16, or between 16 and 20 and in full-time approved education or training.