Original: Their diet seems to be almost exclusively "Tubby Custard" mispronounced as Tubby Tustard by the characters (which is created by a Tubby Custard machine and consumed by sucking through a spiral straw) and "Tubby Toast" (circular toast with a smiley face on it), and they are spectacularly messy eaters.
Simplified: Their diet is mainly of "Tubby Tustard" (which is sucked through a circular straw) and "Tubby Toast".