You would do this by selecting
“Claims for Dependants” under “Deductions”. Answer “Yes” to “Would you
like to review or claim any of these credits?” and click “Continue”. Then on
the “Your children and other dependants” page, click on “Enter Credit Info”
link next to a dependant’s name to select which credits you would like to claim
for that person. The “Children's Arts and Cultural Activity Tax Credit” is
listed as “Children's arts amount (provincial credit only)”