Some time ago, I noticed Jessica had grown a lot in a short period. I told her she was growing like a weed. She thought that was the funniest expression. Recently, we were talking about how much she's grown, and I prompted her "You're growing...."
"Like a seed!" she said. Well, you know, that works too.
Now when I prompt her, I say, "You're growing" and she says, "Like a seed and a weed." She likes to cover all her bases.
Another expression that seems obvious to a 4 year old is the opposite of inside-out. For some reason, I've always said, "Right-side out." I don't even know if anyone but me describes it like that. But Jessica's way of describing it is "Out-side out." I like that better than my way.