If at first it seems so simple, then all of the sudden it is not:
“Under the right circumstances, you are allowed to become a citizen of both Canada and the United States, simultaneously. Many people enjoy the benefits of dual citizenship, allowing them to to travel back and forth freely, vote, and otherwise take advantage of the rights of citizens in both countries.
Before thinking about citizenship, however, you must find out whether you qualify to immigrate to either the United States or Canada in the first place. (We’re assuming that you’re already a citizen of one of these countries.) Citizenship is the highest immigration benefit you can obtain in either country, and getting to that point involves many steps.
There is no separate application procedure to apply for dual citizenship. If you’re already a citizen of one of these countries and become a citizen of the other without taking active steps to renounce your original citizenship, you are a dual citizen. It’s as simple as that.
(Note that taking the “oath of allegiance” to the United States, as described below, is not considered a renunciation of your Canadian or other citizenship.)”
https://www.alllaw.com/articles/nolo/us-immigration/dual-citizenship-united-states-canada.html
So dual citizenship is possible between the U.S. and Canada, and Canada grants citizenship to the children of Canadian citizens wherever they may be born. And U.S. grants citizenship to kids born in the U.S. with regard to the citizenship of their parents (even illegal aliens, as we unfortunately know).
Whether a person is a Canadian citizen by descent depends on the legislation at the time of birth. Generally speaking, any person who was born to a parent born or naturalized in Canada who has not actively renounced their Canadian citizenship is a Canadian citizen by descent (known as first generations born abroad), regardless of the time of birth.
https://en.wikipedia.org/wiki/Canadian_nationality_law#Canadian_citizenship_by_descent