I don't view education primarily as preparation for a later stage in life when education is over. It's easy to see it that way, but I see it more as something like a practice that should have no fixed end. For example, a musician takes lessons, but is already a musician from the time s/he's taking lessons. The professional should still take lessons from a more accomplished professional. The musician still practices even after sh/e has learned to play.
Children playing in the park are doing what they ought to be doing: playing. You could see playing as part of a necessary preparation for being an adult, part of the way the child develops. I'm sure play has a developmental role, but the child is not simply preparing to be a different sort of person, but being a child in the here and now.