What adults should do with their body, is up to them. If they want to mutilate themselves, it is fine to me (as long as they don't harm any one else). But I agree with you on the kids. They should not be harmed. No sex change operation should be performed on the children, until they reach an age where they can think independently about it.
An issue is never as black and white as that. I was friends with an activist with your position who had surgery and was not happy about it.
http://en.wikipedia.org/wiki/Sex_assignment
From wikipedia:
In approximately 1 in 5,000 infants, there is enough variation in the appearance of the external genitalia to merit hesitation about appropriate assignment by the physician involved. Typical examples would be an unusually prominent clitoris in an otherwise apparently normal girl, or complete cryptorchidism or mild hypospadias in an otherwise apparently normal boy. In most of these cases, a sex is tentatively assigned and the parents told that tests will be performed to confirm the apparent sex. Typical tests in this situation might include a pelvic ultrasound to determine the presence of a uterus, a testosterone or 17-hydroxyprogesterone level, and/or a karyotype. In some of these cases a pediatric endocrinologist is consulted to confirm the tentative sex assignment. The expected assignment is usually confirmed within hours to a few days in these cases.
In a much smaller proportion of cases, the process of assignment is more complex, and involves both determining what the biological aspects of sex may be and choosing the best sex assignment for the purposes of rearing the child. Approximately 1 in 20,000 infants is born with enough ambiguity that assignment becomes a more drawn-out process of multiple tests and intensive education of the parents about sexual differentiation. In some of these cases, it is clear that the child will face major discordances or abnormalities of anatomy or function as he or she grows up, and deciding upon the sex of assignment involves weighing the advantages and disadvantages of either assignment.