How can you get rid of frizz in straight hair in 12-year-old girls?

cut it and start to grow it again. Cutting and growing the hair again is of course one solution, but rather drastic if the 12-year-old in question likes her long locks. She could try the methods used by older girls and women, which are mostly a matter of using a shampoo and conditioner for dry hair, maybe even a leave-in conditioner, then dry the hair carefully and use hair straighteners (aka flat-irons) to help get a smooth finish. She needs to use some sort of suitable hair protection spray before using the straighteners, in order to protect her hair from the heat. This advice only applies, of course, if she wants her hair straight. If she has curly hair and just wants it to be less frizzy then a leave-in conditioner or some of the other products which promise to help reduce frizz would help