Building departments use the term “ladder effect” as pertaining to horizontal deck railing systems. Horizontal rail parts are approved in the building code, although they can be climbed over.  Regards any such danger, one must also realize that a chair pulled close to even a vertical 2×2 picket railing can also be climbed by a child.  Each parent must be aware of their children’s inclinations to climb up onto things.  It is good to talk with other parents who have similar deck installations.