I cycle every day to work in Germany and my 3 year old daughter already cycles in the city (not alone obviously). However, I would always wear a a helmet and I think it saved me at least once from worse. I really wonder how Dutch infrastructure could have saved me from getting hit by a 90 year old lady on the way to get doctor on city neighborhood road (was fascinating to see how the helmet absorbed the energy). It might be rather that Dutch drivers learned to share roads with bikes (although I am always scared of Dutch bus drivers when the overtake you on a bicycle)