Suggestion

If the immigration officer (IO) at the port of entry does not allow your parents to use the green card to enter US, normally they will give your parents tow options/choices:
1. abandon their green card at the port of entry by filling out I-407 there, and then the IO will issue the B-2 I-94 to them to enter US;
2. allow them to enter US, but they need to go to the immigration court to depend their green card status. The risk of this choice is that if they loose the case in the immigration court, their permanent resident status will be cancelled and they may be deported.

So, if they have to choose one of the two choices, my suggestion is that they may choose "1.", then you may sponsor them green card again a few months later. If choosing "2.", there is too much pressure on them physically and mentally, especially their English is poor and they have no idea about US law.