What kind of cookie breaks easily into multiple pieces when dropped?

See if you can find a cookie that is both thin and crisp -- the thinner the better. Minor damage will be more destructive on a thin cookie, while on a thicker one it would just cause some crumbing.

• A tuile came immediately to mind as one that gets relatively fragile once it cools. They come in wafers or rolled into cigars. (Pepperidge Farm sells a version, Pirouette, that are filled with chocolate -- but filling might buffer the cookie too much.)
• A pizzele can also be pretty breakable, but when I've seen them in commercial packaging they are a softer version.
• A fortune cookie might be too robust to break on a six foot drop, depending on the angle of impact.

If you decide to make your own, do the opposite of advice to prevent crumbly cookies: use a "softer" flour (cake flour rather than all-purpose, which has less protein), use too much flour, use not enough fat, overbake, and use a dark cookie sheet. These cookies aren't likely to taste quite as good, though, so if your secondary goal is a tasty treat it might not work out!

A spritz cookie would probably work well for you. When made with a cookie press, the shapes have lines indented, which tend to break apart easily.

For homemade cookies, my first choice would be snickerdoodles, and my second choice would be something very thin and crisp like Moravian wafers. The cakey texture of a proper snickerdoodle (made with cream of tartar) makes them prone to breakage, and the fragile, thin wafers break easily.

As you guessed, most commercial packaged cookies are manufactured with being breakage-resistant in mind. The one exception I can think of is Nabisco Famous Wafers, which are quite fragile, and certainly won't survive an unprotected 6ft fall. If anything, you may find the wafers too breakable.

note: I linked to some sample recipes, but have not actually tried those recipes

In principle, what should work best are shortbread cookies, that's cookies made out of pie dough. There is a reason that dough is known in German as Mürbeteig (brittle dough). This dough is made from just fat and flour, with very little water for binding, and it falls apart easily.

I am not entirely sure how well it will work with storebought cookies. I have had brands which are bound much more strongly than homemade dough, and keep together more. So you will have to try your way through them, getting different brands, or really bite the bullet and bake. They are not a difficult cookie to make, although a beginner won't be as efficient as an experienced baker.