Base from my experience when I did my flagpoling, considering that the child is a US citizen, he is eligible to enter the US at anytime time, in my opinion, it is not flagpoling for their is no refusal of entering the US. If you have the visa to enter US maybe you can go with your child and stay for a night then get back to Canada or go to the border together with the child and tell the US boreder security that all you need is to comply the PR status of the child and go back again in Canada. That’s my opinion.