hey, i think i know that one!
hmm... you don't have to buy lots of stuffs. you just need to understand and explain briefly. you can try on a cartolina or an illustration board. draw the sun at the center of it, then create a wide circle around it, that will serve as the earth's orbit. then draw four circles at the four sides. the left, right, the upper and the lower part of the orbit. that will be the four seasons of the earth: winter,summer, spring and autumn and those will also show why do we have four seasons. i think this is the simplest and cheapest way to do it. you don't really have to suffer that much.