Relaxers and permanent straightening chemicals should not be applied to a child's hair if she is under the age of 13, because the hair follicles are still maturing and you could risk causing future problems such as thinness or brittle hair. The fumes of these harsh chemicals are also harmful to an adult's lungs, let alone a child who is still developing. There is a way, however, to straighten or safely and naturally relax a child's hair so that it is more manageable.
Wash her hair with a moisturizing shampoo and treat it with conditioner. Do not massage the scalp in circular motions, as this will tangle her hair.
Divider her hair in sections and comb, starting at the ends and gently moving up with a wide tooth comb. Hold the hair gently at the scalp with the other hand to avoid painful tugging. You may also apply a detangling product, but this is optional.
Use a comb attachment on the end of the hair dryer and dry each section of her hair on a low heat setting. When you are done with each setting, hit it with air from the "cold" setting to seal the follicle and set the straightening effect.
Child Older than 13
Purchase a natural, at-home hair relaxing kit that will texturize her hair to make it more manageable. A chemical straightener is much harsher, but a relaxer will make her hair more manageable so that she can style it to her liking. The child should be mature enough to know and understand the proper after care instructions for her newly relaxed hair before it is done. A "no base" relaxer has the same effect as a stronger relaxer but is gentler.
Apply petroleum jelly to the child's scalp to protect her skin from the relaxer. If you do not have petroleum jelly, make sure you keep the relaxer about 1/4 in. from her scalp. Wrap a towel around her neck and shoulders and apply the petroleum jelly to her ears as well. Most relaxing kits will come with a petroleum cream.
Follow the directions that came with your kit carefully. In general, you should not leave chemical relaxer on her hair for longer than 20 minutes. After you rinse the relaxer out, apply the neutralizer that may have come with the kit, as this will stabilize the chemical residue that may remain in her hair even after thoroughly rinsing. Apply a deep conditioner.
Comb her hair gently with a high-quality comb. Your child's hair will be very fragile right after the treatment, so instruct her to gently comb her hair starting from the ends and slowly moving upward in short motions. After a few days her hair will have healed enough for her to straighten if further with a hair dryer on a low heat setting. She will also need to condition her hair every time she washes it and use a deep conditioner once a week.