"Bath crayons" refers to a type of coloring tool that can be used in the bathtub. These are crayons specifically designed to be used on the bathroom walls or tiles while taking a bath. Full definition
Set sail to distant shores with your little pirate and search for hidden treasure or go on a safari and find jungle animals floating in piles of bubbles or go spelunking and make cave drawings on the sides of the tub with bath crayons. (littleheartsbooks.com)
Bend the rules at bath time with fun products like bath crayons, which allow little ones to create masterpieces on bath walls without permanently marking up the tub or themselves. (verywellfamily.com)
Water - soluble bath crayons are also a fun way to play, but make it clear that they stay in the tub. (babycenter.com)