In the military, we used (IIRC) 3/4 inch white nylon rope and Swiss Seats to rappel. The military style rappelling (where you push off with your feet and do the flying leaps down the side of the cliff) are very hard on the ropes and the ropes were discarded (cut up into smaller sections or simply tossed) after every training session because there was no way to assess the damage. However, for a one-time use such as you describe, this might be a cost-effective option.

Rappeling doesn't take a lot of training, but weighed against that must be the risk of one of your family members freezing on the rope and trapping the rest of you inside. I would think it better to invest in some professional rock-climbing lessons for all of them so they can escape the burning building even if you're on a business trip to Timbuctoo.