I just went through this myself for DD's swimming class. They have youth sized swimming caps at Target but they are the latex kind and hard to get on their heads. I gave up on that one and bought one from the gym where her classes are held that is made of lycra (bathing suit material). The brand is Sprint and here is their website. I haven't found them in any stores, but didn't check a sporting goods store or anything like that. This one works great and is comfortable and easy to get on DD's head with no complaints. Hope that helps!
Last summer, I bought my son's swim caps at www.swimoutlet.com. I bought two - a cute fish swim cap and a cool shark swim cap. The caps come in a variety of colors and styles and are reasonably priced.
Re: Swim hat/ cap - where to find?