In The Road to Hell is paved with Good Intentions a playground expert talks starts with a discussion about requiring helmets at all times and ties it into updated standards for playground surface and what happens in real life. Sometimes these things just don't work out like expected. On children wearing helmets:
The question arises then, what is the impact of using a safety helmet? In talking with child development physiologists they suggest several issues. First they suspect, although there is little research on this, that such protective gear may disrupt the normal progression of reflex maturation. They also are concerned that the lack of consequences when falling may retard the child’s ability to form proper assessments of their skill, i.e. reduce their judgment. Finally they speculate that it reinforces a pattern of parenting that is over protective and ultimately harmful.
There are many possible reasons for the accident rates to remain unchanged that have nothing to do with the resiliency of the landing surface. One possibility is that the recent trend to cover the entire playground with rubber is so expensive that the amount of play equipment has to be reduced significantly. This results in a lack of events, which in turn reduces the opportunities for graduated challenge that allow kids to gain skills incrementally. On the one hand little kids must use equipment that is beyond their skill level and on the other hand older kids find little to challenge them and so use the equipment in inappropriate ways.