have you tried creating each part (car & marshmallows) separately and adding them to an Assembly? you would create 4 separate parts (or 2 parts if all the marshmallows are going to end up identical) You can then mate and edit each part inside the assembly to get what you want. There is an option inside the Assembly to JOIN the parts and create 1 solid model if that is what you're trying to end up with. you can find instructions for the JOIN option in the help section. i could tell you but i find it better to learn this stuff from a hint - i'm mostly self taught myself.
good luck Bill