"But they would be non resident if living and working in Canada (their original country), yes?"
With regard specifically to your California state income tax obligations, yes, you would file your California state tax return as a non-resident of California.
Your federal tax obligations to the U.S. and/or Canada are a separate issue, and would be determined by the laws of each country and by the U.S. - Canada tax treaty.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
