This is how I made it. The "graham crackers" and the "chocolate bar" are all made from 8 inch square cakes (the chocolate bar was then cut smaller so it would fit at an angle nicely) and the "marshmallow" is a 6 inch round cake. The hard thing I think were the "graham crackers". I didn't want them to be too tall or they wouldn't look like graham crackers, but I didn't want them to be too small cause who wants a short tiny piece of cake? This is how I solved it (don't know if it was the best decision, let me know in the comments how you would do it.). I took the bottom "graham cracker and the chocolate and crumb coated them together. Then I covered the entire thing in the graham cracker color. Then I put the chocolate fondant on top of the chocolate bar one and was just very very careful as I trimed around it so as not to cut the graham cracker fondant below (I had a couple tiny nicks but not too bad). Then the top "graham cracker I baked a little taller cake so they would still get enough cake to satisfy them. It used up a little more fondant and I was nervous about the trimming, but I didn't want both the graham crackers and the chocolate to be short little pieces of cake.
And this time I enjoyed making the person!!! YEAH! I did a little different route. The nose is still the same round little ball, but the eyes were made with a food coloring pen and the mouth was painted on with a tiny tiny paint brush (make sure the gumpaste has some time to dry before adding the coloring/painting or it will indent the paste as well as color it and it doesn't look as nice. It could also be that I love the little girl that the cake is for so making her was more enjoyable (sorry Sleeping Beauty).
For the graham crackers I used a ball tool to make a slight indent and then a little paint brush to make the hole a little deeper. And a gumpaste tool to make the dotted line that you break your cracker at. I used charcoal petal dust (not real charcoal just the color charcoal) to make the marshmallow look a little roasted. And I used my cricut cake machine to cut out the letting for the cake.
Here is a little better angle to see the s'more and Sammy roasting a marshmallow.