I can't figure out how to edit a post. Maybe I can't.

Here's the answer about a tax year when a child dependant turns 18.

It seems if they turn 18 that year, the whole year is not usable as a dependant, even the portion of the year they were 17. Terrible.

Pursuant to subparagraph 118(1)(b.1)(ii) of the Act, only an individual who is the parent of a child who is under the age of 18 at the end of a taxation year who claims the equivalent-to-spouse credit for a wholly dependent person in respect of that child, can claim the child tax credit in respect of the same child.