The easiest way to introduce 2D and 3D shapes to kids is Flashcards. These flashcards are showing 2D and 3D shapes with a description of those shapes to make it understand what is the difference between 2D and 3D shapes. For small kids, the introduction is just through visualization means showing the cards but after some years you can use the same cards to explain the characteristics of the 2D and 3D shapes.
This set contains 18cards in a set.