When I make figures for my cakes and cupcakes I don’t like to use solid fondant or gum paste because they become SO heavy. I like to make a rice crispy base that will be hard enough to support but not as dense. And for some reason kids think they are the most fun to eat!
I make my rice crispy treat with 6 cups marshmallows, and 6 cups rice crispy cereal, and just 2 teaspoons of butter in the pan or bowl to help with the stickiness.
This will be a lot dryer than the treats you make to eat, and that is the goal. You don’t want something that is going to stay really moist. As I make the inner shape of whatever character I sometimes roll in a bit of gum paste powder if I want it really firm. If I want the character smooth like this Lego Topper I roll the rice crispy really tight on a cutting board, and if I want a bumpy texture (like Rex) I just press more lightly. I build my shape around popsicle sticks or wooden dowels then let them sit overnight to harden up before covering with fondant.
If the figure is still to bumpy I put a VERY thin layer of Wiltons fondant over before applying the color fondant I want. For these types of figures I use Homemade MM Fondant. Really the sky is the limit- you can make anything!