For smaller wrists, the Nike Sports band has the most holes, so is more likely to fit than the regular sports band. The solo loops are very light and comfortable, but you'd need to take your niece to the store and try them on to find the right fit. Also, they tend to wear out and break after a while -- mine lasted about nine months. But it really was super comfortable while it lasted. So if you don't mind having to replace them when they break, and there is a size that fits your niece, that's the most comfortable band for someone with small wrists.