RM,
Not exactly the story you were hoping for, but relevant nonetheless:
Years ago, we were out riding with the kids and SilverDad (at about age 70) said "I want to ride your bike". I was scared, because he's not very coordinated, but he was determined.
He got on the bike and rode half a block - wobbling and shaking the entire way. I was freaking out.
Needless to say, I didn't encourage him any after that.
Unfortunately, today, he's 80 and often doesn't have the ability to even get downstairs to dinner because his legs are so weak from his sedentary living...and that's using a walker!
We had them in exercise classes at one point, but they didn't stick with them.
I'm glad to see what your parents are doing...and remember, your kids favorite toy is a cardboard box (not the expensive item that came in it) and I bet your parents will love their bikes.
If you don't grow where you're planted, you'll never BLOOM - Will Rogers