I always allow students the freedom to come up with their own idea which
fits within the perimeters of the technique I am teaching. I find that
elementary students are like little sheep--they follow wherever you lead--so
if you tell them we are all going to make turtles then they all fall right
in line. However, if you tell them we are going to put pattern inside an
animal-please think of an animal that would work well for this-your animal
needs to be different than everyone around you---they will then really
brainstorm and come up with some unique ideas most of which I never even
thought of. That to me is the beauty of art.
I would encourage you to restrict it a little at first-- for example--
restrict to animals but not the specific one. If you are too broad (draw
anthing you want) they often lose the concept that you are trying to teach
or what they pick doesn't fit the lesson and they don't feel successful.