Yes, cosmetic surgery should be used to help children with body image issues, because they can help children that are severely deformed. If a child has a birth defect, they will never feel comfortable socializing with others until it is corrected. It is fair and helpful to the child to correct legitimate birth defects.
If the child doesn't like what they see in a mirror image, or is at a health risk due to unhealthy habits and unhealthy body weight, then this child should be coached on healthy and natural ways to improve their health. This is a proved method of improving health and self image. If the body image issue is formed by believing the opinion of others as fact, then no amount of cosmetic surgery will ever help the child , or any other person improve their self image.
No, surgery isn't necessary for body image issues. Children with such issues need consoling on how to improve their self-esteem and how they view their physical condition. Altering their bodies simply for superficial issues only deepens the child's own negative view of their body. This sort of behavior only futher acerbates the body image issues.
It is clear that we have gone too far as a society when cosmetic surgery is seen as a viable option for children. In the United States, there is a significant body image issue, one that has even begun to effect children. But we should protect them from such fears, rather than perssuring them further.
I do not believe cosmetic surgery is the appropriate way to address body image issues, especially for children. Body image issues are a mental problem and they should be addressed as such. Cosmetic procedures can be dangerous and they also don't last a life time. Providing these services to children would be an injustice to them.