This is just anecdotal evidence, but I recall being zapped on more than one occasion while growing up. None of those zaps were due to sticking a foreign object into a receptacle.
It is already somewhat difficult to seriously harm yourself by that that way. Generally, if there is a good ground nearby, there is already a requirement for a GFCI. So you typically need to simultaneously push two metal objects into the hot and either the neutral or ground to hurt yourself. It would take a bit of doing for a two-year-old to do that. Not impossible by any means, but it's not something that has a high probability of occuring.
And if said two-year-old is pushing two things into the receptacle, I believe that they are designed to open in that case, because it is indistinguishable from plugging in a plug.
