I recall that the nominal capacity of the human body to earth is when the person is standing in free air. Laying flat the capacity to ground is several times the nominal. I would imagine even a small child laying flat on a slide would have a capacity to the slide greater than the nominal 100 pf.the generic 100pF assumption may well be high for the small children likely to be using the slide.
When the child stands up his (her) capacity relative to the slide reduces by many times so the potential increases.
V = J / C
where J is the static charge which remains constant until the discharge occurs
So discharging the child while still on the slide will be a lower voltage and therefore less noticable shock.