Downsides to US-Canadian dual citizenship for US resident?

If they’re us-resident there should be no problem, assuming the person travels on the us passport outside of Canada. They’d have to travel with two passports going to Canada, as each country expects to see their passport upon entry. Canada doesn’t tax non residents like the us so there’s no tax changes without moving into Canada, and there’s currently a tax treaty for that.

Currently, none as far as the US is concerned[0]. The child is a US citizen and while in or entering/exiting must use their US passport. If they move to Canada (or any where else), they are still required to fulfill US tax obligations.

You'll want to check if Canada has the same tax requirements for citizens living in a foreign country. In Canada, the child would be a citizen of Canada and US consular protections/services are unlikely to be helpful there. Same while in the US, they are only seen as a US citizen and are subject to the US.

However, a bill was introduced: Exclusive Citizenship Act of 2025. You may want to follow it (in-case it actually passes and is not simply grandstanding for maga). It would require US citizens to only hold one citizenship and dual nationals would be forced to renounce any others or automatically lose US citizenship. Which would be challenged up to the Supreme Court as US citizenship is protected in the Constitution and can only be voluntarily renounced. The provision saying it would be lost by missing the one-year deadline to provide a written renunciation of their foreign citizenship to the State Department -- is very likely unconstitutional.

[0] https://travel.state.gov/content/travel/en/legal/travel-lega...