I know people do it, some here said they do. My opinion and a lot of others as well, is NEVER take a kid on a mower with the blades engaged. NEVER. No amount of prevention can prevent all accidents. All it takes is once. I have a friend who fell off a rider and was run over. He was 4. He could hold on, as someone said, but apparently not good enough. He now has a prosthetic leg.
This tread reminds me of a safety ad Deere put out a while back. they say "How do you stop a 200-mph blade?" "You dont."
The danger is enough without the blades moving.