Here is a super easy bookmark craft that is really simple and cute.
glue stick (or glue gun if you supervise)
wooden animals or other embellishments
Trim the cardstock to desired size. These are typical bookmark size. Lay out the design and glue as needed.
These little critters are from Hobby Lobby. If you want to go a less expensive route (altho the little critters are only about 59 cents each) you could draw/cut out your own animals. These are so inexpensive tho and will make the bookmark more sturdy.