It almost sounds as if the "parents" are more concerned about the decor, than they are about the children. :-( I would not make a black & grey quilt, for any young child. I'd go with my instincts and make it colorful; if the parents have a snit about it, just offer to take it back. . . and offer them maybe $10 as consolation (maybe they can find some dreary, old rag for that much).
