What about Helmet Covers for Kids' Helmets?
There are some really cute helmet covers and helmets for kids (and adults) that have horns, ears, and whatnot on them. While the idea of looking like a dinosaur or animal while out riding does have a certain appeal, we agree with the general advice that helmets should not have any sort of projections. Modifications to the helmet, including outside add-ons, can change the protective charcteristics of the helmet. The helmet is tested against certain standards, and it is tested without any add-ons. Although they tend to be soft, there's no guarantee they will break away or not get caught on something and violently jerk the helmet. Having an additional feature sticking off the helmet can change how it behaves in an accident, therefore we don't recommend them.
If you're curious, the CPSC guidelines have this to say about add-ons/projections on helmets: "A projection that extends more than 0.28 inches (7 mm) from the outside surface of a helmet must break away or collapse when you hit it with the same force that the helmet receives when you test it for impact attenuation. The inside surface of a helmet may not have any projection that extends more than 0.08 inches (2 mm) inside the helmet."