I assume they don't meet the RO? If that's the case, your options depend very much on why they didn't meet the RO.
If your wife has mitigating circumstances like having been outside Canada in order to take care of a sick relative such as an ailing parent and she can prove it, she can apply for a PR TD on humanitarian grounds and they will most likely get it, can return to Canada and renew their PR cards.
If she has no mitigating circumstances, she can apply for a PR TD too but it will most likely be refused causing her to lose her PR and you would have to sponsor her.
If she and your son fly to the US and cross at the border with their expired PR cards, they will most likely be reported on entry for not meeting the RO. They can appeal and your son will likely be given a pass based on being a minor and your wife could lose her appeal if she doesn't have a good reason for not meeting the RO. Hence it could happen that after 1-2 years in Canada, she loses her PR and is asked to leave and you would then have to sponsor her.
Even if they don't get reported, life in Canada without a valid PR card can be difficult as they work on reaching their two years so they can apply to renew. You could have problems with health care, you could have problems enrolling your son in school, your wife could have problems getting a drivers license and if she doesn't have a SIN already, she wont be able to work.
Ultimately it's up to you but if they renounce their PR status based on not meeting the RO, you can sponsor them for PR again and there are no consequences to them for having lost their PR that way. They will be approved again as long as they pass medicals and background checks.
