I once taught a successful class to second graders, on euler's theorem for polyhedra by handing out colored cardboard models and letting them count the facets, etc... This would seem a suitable topic also for elementary school teachers.

(When reading elementary presentations of triangle geometry for children one sometimes encounters false statements, such as the claim that the rigidity of a triangle made of straws implies SSS congruence. Of course rigidity implies the moduli space is discrete rather than that it is a singleton, i.e. a similar proof shows (the false theorem) "SSA congruence".)

Another favorite topic, harder than euler, is modular arithmetic applied to computing what day of the week someone was born on.