I am not sure if you are asking about psychological preparation or physical preparation in anticipation of surgery.
In the former, probably the most powerful influence will be the child's desire for surgery. If they are self conscious of the lip and constantly looking in the mirror then they may be eager for surgery and will require little mental preparation. If, however, they are pleased with their appearance they may take your insistence as a sign of disapproval.
In regards to pre-operative preparation, the surgeon's office should be equipped to provide you with sufficient information and instructions to adequately prepare her.