Thank you, dear 863211.
One more question:
After submitting the I-485 in the US, the kid will need going back to Canada for studying. How long should the kid stay in the US for the I-485 submission purpose?
-After USCIS receive his/her I-485. So, it should be safe to leave after you send out your I-485 for 3 to 4 days using Overnight Express mail.