We returned in 2013 and I got pregnant soon. After the child was born he had severe asthma which appeared mostly on cold weather , and a doctor recommended to wait some time before moving to a cold climate ( then we didn't know about the future). The asthma became better as a child grow up and now the child is healthy.
What can be a worst case scenario in our case if we entrance from US border ?
It could be seen as your personal choice to return to home country and have your child there. You could have also done so in Canada.
Worst case if you attempt to enter Canada via USA land border, you could be reported by CBSA if they notice you are not in compliance with RO and they don't accept your H&C reasons. If you're reported you can still enter Canada, but will need to appeal the decision. The appeal date may take 1-2 years, during which you can continue to live in Canada as PR. However if your PR is denied, your PR status is then terminated and you will be asked to leave Canada.
If CBSA does not notice your RO violation and lets you enter Canada, you will need to stay here for
2 straight years without leaving for any reason whatsoever. Only after 2 straight year here will you be back in compliance with RO, can renew your PR card, and can apply to sponsor your child for PR.
Note that with a valid PR card, there is no reason to enter via land border you could just as easily fly here with a valid PR card and your son with eTA. Same possible scenarios would apply at the airport or land border.