"I can't!"
Zion is a very capable child. He can do many things, and he tries gamely to do many things for himself that he can't actually manage. Despite that, one of the phrases we hear most often from him is "I can't!". I wish I had audio to give you the full effect; it's kind of a whine, in that the vowel sound is drawn out, but given the context it's generally not annoying, because mostly it's true. He is very small, after all. It's often rather endearing, actually.
Even better than the plain version are some recent extensions. Food he prefers not to eat? "I can't like it!" Would rather not have sunblock on? "I can't want it!" I'm not sure if he's aware of any semantic distinction between his speech patterns and a more traditional formulation—using "don't", probably—but it does make me wonder if there's any psychological insight to be gained from analysis of his unique syntax.