I've been playing this game with my 2 and 4 year old kids.  They both really like it, and grasped the concept quickly.  I adapted the game for younger players by leaving out the muncher cards. Also we say the name of each card out loud as we lay it down so that I can use my voice inflection to cue them that it is time to slap (...tomato...hot dog...bacon...Bacon!)  My 2 year old loves to slap the sandwich thieves and say, "stop thief!"  She is too young to recognize the sandwich patterns to slap, but still has a lot of fun playing.  My 4 year old, after playing a few times, has learned to slap the doubles (2 of the same cards in a row) and is just starting to recognize the sandwiches (3 cards: 2 of the same with something different in the middle).  He has also become quicker at recognizing and slapping so that he can do it first.