I don't think an iron would work bazzard, perhaps a hairdrier would be better and keep dabbing with a tissue when it melts. Another method to try is baking powder on a damp cloth using gentle pressure, but don't rub the stain as the wallpaper will come away. Lakeland make a stain removal block that is very good for walls but I'm not sure if it removes crayon you would have to check.Good Luck.