Children are too young to make decisions on how they think they should look, and their bodies are still developing.
In the case of things like cleft palate, facial tumors, reconstruction after accidents, and other medically useful or necessary operations that require plastic surgery, doctors should be allowed to perform such operations.
Firstly, sorry for the mistake - the question should have read 'plastic surgeries'. There has been a worrying trend of parents having their kids go through plastic surgery these days. I think it absolutely has to be banned because children's bodies have not maturely developed and these efforts to mess with their growth likely won't have positive effects on physical health. Even if they aren't harmed physically, young children who are blindfolded during a surgery often think they're going through an operation to cure some serious disease. This can be traumatic experience for them. Plus, it will give the child the impression that (s)he was born ugly and (s)he will likely value physical beauty more than inner beauty as (s)he ages.
I agree that performing plastic surgery on children should be discouraged but not banned outright. Plastic surgery, while abused by many, can be lifesaving after accidents. What about the plastic surgery performed on children with cleft lip deformities? What about the plastic surgery performed on children who are bitten or attacked by an animal? Plastic surgery should be an option only when absolutely necessary.
Whilst elective cosmetic surgery should be strongly discouraged or even banned, plastic surgery is an umbrella term used to describe reconstruction of topical and even lower level soft tissues. It would be reprehensible to risk refusing children actual medical care because the term plastic surgery is typically used to describe superfluous surgeries that people elect to receive to change their aesthetic appeal.
The burden of proof is on the people who want to outlaw something that is currently legal. People often get plastic surgery for stupid reasons, but that's not a good enough reason to ban something. I've heard about people who got their faces messed up in an accident to the point where they are unrecognizable and they get plastic surgery to remedy this.