A Shrek party sounds like great fun.
If the kids were older (like 6 or older) I would say, get "gross" with the snacks. Anything involving slimy green stuff
But you are talking about little ones.
Green jello, or vanilla pudding colored with food coloring would be good.
If they don't like gingerbread flavored cookies you could use a sugar cookie recipe, mix in 1/4 cup of baking cocoa, and make chocolate men.
Otherwise I wouldn't worry too much about doing theme food at this age. Just pick easy stuff that they will eat (chips, hot dogs, chicken nuggets, vegetable slices like carrots, cucumbers, etc.) They are little enough that they might shy away from anything that is "different" in the way of color, flavor, texture....
Have fun!