The child tax credit, going back to a statement that Louise made earlier, is one of a number of adjustments made to ensure that income is properly recognized in determining taxes payable. That's the nature in which it was introduced. As a result, it reduces your taxes payable.
For a family now earning $21,000, because of the generosity of the existing basic personal amounts available to them and dependants and other credits, they're generally non-tax-paying at that level. As a result, they wouldn't need the credit, as they're not paying taxes.