This is a serious question. I know other characters have had cereals over the years.
I think it would be easiest just to have a "Thing crunch". It would be difficult to capture the essence of all four characters. I suppose you could have the previously mentioned Thing Crunch with blazing marshmallows. A toy could be a message that would need to be decoded with an included invisible ink pen. I don't think you'd want cereal that stretches in your mouth. Perhaps just a random celebrity on the front of the box saying "That's Fantastic!"? Or "It's swallowing time?"