How To Get Rid Of Black Marks In Thighs?

i have black marks between my thighs since i was young, and my 8year old son also has the same ploblem.this makes me so embarassed whenever i want to go swimming or when i want to have sex with my husband. what can i do to get rid of this problem?am 37 years old, 2.6 meters hight and 81kgs. for my medical history i dont have any problem