I live in Quebec, have 50% custody of two children and paid their mother child support in 2018. Is there anywhere I should be declaring that?

[TurboTax-Marie-Paul]

Yes you should be declaring that - here is the easy way to find where to enter it:

On the top right corner, there is a search option (magnifying glass)

Type in Alimony

Select & click go - it will take you to that section to enter the amount.