Can these be placed outdoors? 

Yes, but like anything made of wood (maybe your deck), you must maintain it
by brushing on multiple coats of a waterproof finish (yearly).
I typically brush on two coats of a polyurethane.


Will my snowman crack?  

Similar to cement, this is typically what logs do. 
Like wooden floors, the spaces between the boards may open and close
depending on the amount of moisture that is in your environment.
Sometimes I fill cracks and sometimes I don't. 
Some people see cracks as flaws while others see them as character.