I want a book cover that brings back the feel of the old Nancy Drew and Hardy Boys canvas painted covers of the '30s and '40s, most notably the covers by Rudi Nappi. http://hardyboys.us/hbart.htm
The main characters in the story are Mai, who is a small, Asian girl of Chinese decent with past the shoulder length straight black hair, Rupert, a slightly overweight red haired boy that's only slightly taller than Mai, and Janie, a tall, athletic girl that wears her mid back length brown hair in a ponytail. Mai would typically be seen in a t-shirt and jeans, Rupert in a button down shirt and Janie is more athletic, sporty looking clothes.
There are a number of elements in the book that the cover could use to generate excitement for the story, including an underground tunnel that the children must escape through, the glowing green face, a tunnel cave-in, darkness and flashlight beams and walky-talkies.
The mask is described as "A green, floating face with wide, black eyes and a melting mouth". The tunnel is carved out of the earth, held up periodically by rotting wooden beams that go up the side of a shaft and across the top, like an underground mine you see in the movies.