reading and writing are not the same as riding a bike. a human needs to read and write correctly in order to function and succeed, but they do not need to having "proper riding technique" -- they do, however, need to have fun. turning a fun activity into the next violin lesson is simply bad parenting. the most important thing to teach a kid in regards to bicycles is how fun they are. pushing "proper riding technique" is on sure way to alienate a child from bicycles.

and I think the idea of even bringing up "proper riding technique" with a child seems like such a downer. do you honestly believe any 3rd grader actually cares about their spin and what muscles they're pedaling with or if they're getting aero enough? I think they're more concerned with going fast down the street, ghostriding their bikes, and jumping off stuff -- as they should be.

as far as foot retention is concerned: they could easily have their foot slip off the pedals from being wet and have the pedal come around and entangle their leg. a crash and/or injury would probably ensue.

a freewheel or coaster-brake seems to be a better and more obvious solution. I see no benefit from a parent forcing a child to ride a fixed-gear other than some weirdo "proper riding technique" obsessed parent or some sort of hipster mini-me thing.