When I was still a child, I dreamed of being a doctor.
or
When I was young,I dreamed to become a doctor.
or
When I was only a kid,I had a dream to become a doctor.
When I was still a child, I dreamed of being a doctor.
or
When I was young,I dreamed to become a doctor.
or
When I was only a kid,I had a dream to become a doctor.