If your parents need an aisle seat, then you will need to pay the seat selection fee. That is the only way that they will be guaranteed (mostly) an aisle seat. If they want to be seat near each other, you have the advantage of selecting seats across the aisle or one behind the other. And in the section of the plane that is most comfortable for them. In other word not all they way at the back of the bus.
There is a chance that they could get assigned aisle seats at check in. But there is a greater probability that they will get assigned middle seats somewhere.