Snake Snack
Use unbaked French bread cans(avaialable near biscuits in dairy section of grocery stores) roll out like a snake, sprinkle cinamon on snake to make snake markings, and bake per directions on package. Use two olives for eyes and red peer for mouth of snake. 
Cactus Garden
Buy several cactus from a local store and let your children explore them. Be sure to tell them not to touch as cactus can hurt. 
Desert Jackrabbits
Give your children Rabbit ears and let them pretend to be jackrabbits hopping
around the classroom.
Pantyhose Snake
During reptile week put pantyhose snakes in the play are. They're easy to make
just put tissue in one leg of pantyhose. Add your own facial features to make
the snake look more real.
Desert Snack
Sprinkle graham cracker crumbs on pudding to represent sand, put in licorice cactus, and other fun items