Dear Madam Doctor:
We are delighted to receive your inquiry. You are obviously an ambitious student with definite goals. Being ambitious and having specific goals is half the battle. Stay motivated, stay focused, and work hard; then there isn't anything that you can not achieve.
Through your O-Level (ordinary level) and A-Level (advanced level) studies, you should take courses in biology, physics, chemistry, and mathematics. The knowledge of biology, physics, and chemistry is fundamental to medicine. A certain level of knowledge in mathematics is also necessary.
Once you are in the medical program, you can plan to do your specialization in Pediatrics. As a children's doctor, you would be called a pediatrician.
Another piece of advice: do some volunteer work in a hospital or clinic, ideally with pediatrics doctors and nurses. (Of course don't let it interfere with getting strong grades in school.) The experience has many rewards. You will begin to understand the everyday life of medical care. You will begin to discover if this really is what you want to do. You will have something powerful to put on your application to medical school, other than your stellar grades. You will experience how it feels to help other people.
Madam Doctor, we wish you the very best!