What creates rigidity in people? Tension.
Logically, then, if one wants to recover the spontaneity of children, the ability to live in the moment, to be free, then the only path is to unlearn chronic patterns of tension.
I will say again that Tarthang Tulku’s Kum Nye system is the smartest system of personal cultivation of which I am aware. Some of the crazier of us will also need Neurofeedback, but I think the two together constitute a path back to a world most of us have given up on.