A cotton swab with a tiny bit of regular old isopropyl (rubbing) alcohol is what I use to clean video game cartridges and consoles, and I've used them before as well with tamagotchi. Just make sure they're all dry if you take them apart to clean them
Edit: make sure you don't rub TOO hard on the shell designs/the shiniest parts of the shell. Just a pretty quick once-over should remove any dirt or unwanted stuff, but if you start to rub hard I've noticed it kind of wears the finish off and will eventually pull the designs too.
