Buttercream icing is often popular with children. I often ice these semi-frozen. It helps with the crumb and getting a smooth finish.
Depending what kind of cake you make, you might not need to freeze it if you are making it just 2-3 days in advance. A chocolate mud cake might fit the bill here. If you contact a professional cake decorating store they should have a good mix you could buy and they can advise on how long it will keep. Some professional cakes can take a couple of days to decorate, so the cakes have to last.