When I was an undergrad, I heard a story where a young child was excited by watching 6 equal-sized equilateral triangles fit together to form a regular hexagon. I don't remember what her age was, but this sounds doable for 5-year-olds, especially if you make the triangles take the colors of the rainbow, excluding indigo.
This is exceptional, in that this is the only example of a regular polygon decomposable as the finite disjoint (except for boundaries) union of smaller regular polygons of a different shape. If one drops the "different shape" requirement, one can put equilateral triangles together to make a bigger equilateral triangle or put squares together to make a bigger square.
But 5 may be too young to get a feel for how, for example, angles work. I don't know how they'll handle failing to put equilateral triangles to make a square, for example.