Legal fees regarding child custody and establishing child support

I really don't think so, check this out:

• You cannot claim legal fees you incurred to get a separation or divorce or to establish custody of or visitation arrangements for a child. For more information, see Guide P102, Support Payments

But there are a few exceptions:

http://www.cra-arc.gc.ca/E/pub/tp/it99r5-consolid/it99r5-consolid-e.html#P137_16374

see paragraphs 17 - 21

Yes you can. Line 23200 for Canada 2020.

The line may change from year to year. 

In case, do a google search "claiming child support legal fees line number" it'll pop up.

https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/tax-return/completing-a-tax-return/deductions-credits-expenses/[product key removed]-deductions/[product key removed]-fees.html