I do not make them wear helmets in the backyard - there is not enough room for them to get up to any serious speed, there is no risk of a VEHICLE hitting them and the 'fall distance' off the small ride on toy or trike is LESS than if they fell off the climber or a chair and I do not make them where a helmet engaged in those activities.

Now if I felt the 'desire' to allow it and they were riding their bikes off my property where we were crossing driveways, roads or out where vehicles traveling at higher speeds were driving than yes i would make them wear a helmet than because my concern would not be a fall from the bike but getting HIT by a vehicle where the driver had lost control for some reason OR a child loosing control of their trike and veering into the road and so forth than they would be wearing helmets for sure ... however I would not be comfortable with taking multiple children out and about like this so we 'walk' anywhere we go and they are attached to me crossing any streets and driveways and so forth.