Red Cross has certifications for swim instructors.
I am a WSI so red cross swim instructor and I would suggest as the person before me as well to take that class it teaches you the proper was to teach stoke progressions and techniques you will need as well as dealing with different ages. you will need to be a good swimmer for this course and already at least know the stokes. Freestyle, backstroke, elementary backstroke, breast stoke, side stoke. butterfly, diving from the deck at least. so you at least have a heads up. good luck!
There is also this company that provides education programs and lesson plans.
Red Cross is the swim lesson leader in the industry. They do require some support training as well. I also recommend becoming life guard certified. Some facilities actually require all water program instructors to be life guard certified. And being prepared for any possible emergency is never a bad idea. Good luck.