First a little exploration is required to find objects that will make interesting rubbings.
Take, for example, the center of this chair:
A crayon rubbing from this source may be used to create the following:
Younger children may be better off with an actual kit that contains ready made texture tiles for rubbing crayon colors onto a page. My sister sent us such a kit with waves and so on to rub patterns from. Or one can collect simple things for children to rub from to create something like this picture from a jar:
