Oooh, get you. Doctor Nick!
Agree. Whilst our roads in Derbyshire resemble and off-road course most of the time, it's not so much the jolting and banging up and down - it's the being thrown forward that's dangerous. If they are still in a rear facing seat, it's because their neck muscles need the support. Once they go into a forward facing seat, it might be a different matter. But I'd still not go extreme until the point that a child understands the words "Brace" and "Oh s***!" to the point that they can actually brace themselves.
Chris