Re: Aikido. The Martial Art.
Yup, they'd be fine.
You're severely underestimating that resiliency of a motivated person. Go to any skate park in the US and watch kids just EAT pavement, with no protection gear, from 10-12 feet in the air. Some of them will curl up, cry, and be hurt. But the majority of them will get up bleeding, limping, and laughing because they just don't give a f%#$* and can take it.
I'm also with Don, RE: heel hooks. They're regularly way more dangerous than anything in Aikido.