Where do I add in my children's activity credits? I searched it but the link it goes to only talks about tuition and education amounts etc

The federal activity credit and many of the equivalent provincial credits were discontinued for tax year 2017.

If you live in BC, MB, QC or the Yukon, you'll enter your activity amounts in the Dependant credit section.

From the navigation menu across the top of your screen when signed into your return, select Deductions > Claims for Dependants.