The tax credit for an eligible dependent can only be claimed once. Each household is allowed only one claim for this amount, even if there is more than one dependent in the household.
You should enter all your children and other dependents when preparing your tax return to claim other dependent benefits that may be available.
All the children should be claimed under Dependants. Click on "FIND" on the top right of the screen type in Dependents and click GO- this will bring you to the dependent screen where you should have entered the children's information.