My Child's Upper Lip Is Discoloring. She Is Only 8

My child's upper lip is discoloring. She is only 8 and has not done waxing treatment, bleaching, smoking or drinking coffee which seem to be the leading cause of lip discoloration. It has now grown to 1-2mm wide from top to bottom. What is the cause for the discoloration?

Also, a website mentioned that it may be oral cancer. But, she regularly visits the dentist.