That playing with blocks is a precursor to theorizing about what exists does not justify a field that purports to make true statements about what exists. Similarly, throwing a football around does not justify a field that purports to describe the behavior of entities in the world. These practices need external justification. The point is that the substantive content of the field of metaphysics has no such justification.
Uh... but "shapes" existing is literally ontology. It's discussing the existence of non-physical entities in the world, and children first come into contact with that by matching the positive version of the "shape" (block) with the negative version of the "shape" (hole), abstracting the "shape" from its literal manifestation.
Discovering shapes is implicit ontology in the way throwing a ball is implicit physics.
