You're WAY overthinking this. I see this is your first post. Have you been doing CrossFit yet? If so, you'll know the benefits first hand. Don't deny your parents these benefits. Everything stays the same, it just gets scaled.
For ideas on scaling, check out Brand X martial arts (WFS): http://www.brandxmartialarts.com/
From "What is CrossFit":
The CrossFit program is designed for universal scalability making it the perfect application for any committed individual regardless of experience. We’ve used our same routines for elderly individuals with heart disease and cage fighters one month out from televised bouts. We scale load and intensity; we don’t change programs.
The needs of Olympic athletes and our grandparents differ by degree not kind. Our terrorist hunters, skiers, mountain bike riders and housewives have found their best fitness from the same regimen.