Not "misuse" - you should not put a child in range of an air bag period. If you have any choice. That's the law, and the law is based on sound medical knowledge and case studies.

If you have to put a child in the front seat having an airbag, please take additional cautions. Even with additional precautions there is an increased risk to the child, no matter how "correctly" you use it.

Speaking of "correct" use of safety equipment, back to the helmet. Furthering the analogy, "misuse" of the helmet in my opinion would be allowing it to unnecessarily contact the ground. So I'll refresh my question, to anyone who has relevant experience or specific knowledge: if you've had formal training in rolls and break-falls, have you found the helmet to be an impediment in the technique?