They must pay the GC fee of $220 each .. recommended doing so before they come. They will present their passports at rntry and the CBP Officer will access their files electronically and they will enter the US. They are LPR as soon as they enter. The visa page will be stamped and this becomes their temporary GC until the plastic cards are received ( weeks to months). They can reenter the US, get their SSN, drivers licence, bank accounts, health insurance etc using the stamped visa
As they are now LPR, my understanding is that they are no longer residents of Canada for health insurance purposes. I hope you are aware of how to get them coverage here.. and the challenges associated with that