Have you considered having children? These rugrats will give you hours upon hours of entertainment, as you place them in situations which may cause them bodily harm or emotional distress!
I think this was intentionally orchestrated to ensure that the kid ended up crying after a hi-larious "blooper".. I mean, even if the whole "rolling your kid down two planks of wood at a steep incline" plan had gone off without a hitch, the kid was aimed at the back of a car that couldn't have been more than four feet from the end of the homemade ramp.
I choose to believe that the kid took at look at his family, judged the risk, and then tricked them into going along with this stunt, while taping it, in order to win his freedom.