Words
The younger one is fond of making up, modifying, or misunderstanding words:
"Stupidious." Not sure how this is different from just "stupid", but usage implies a stupid that is more than stupid.
"Overreacted." The state of having overreacted to something. Unfortunately I'm most often the target of this one. "Don't do that or Dad will get overreacted."
"Halfwise." Apparently the same as or simply a failed attempt at "halfways".
"Dim." Dim (and bright) either metaphorically (or perhaps in his mind literally) correlate to dull and sharp. He will sometimes describe his teeth as "feeling dim" if it's late in the day and he hasn't brushed them.