Although I have been known to act like an evil Grammar Nazi from time to time, I usually try to correct WCK's grammar in a subtle, non-obvious way. I will usually repeat the correct word(s) back to her, as though it's just part of the conversation. For example, if she says something like, "I runned really fast!" I'll say, "Really? You ran really fast? That's great!" And so on. I figure it will just sink in after a while.
However, there is one thing that she does that I've never corrected because I find it so dang hilarious and cute. According to WCK, multiple articles of clothing are called clothes. If you have a single article of clothing, such as one shirt or one dress, it's called a "clo."
Really, this makes perfect sense.
She'll say things like, "Can I get a new dress-up clo?" or "Maybe we should get Daddy a new exercise clo for Christmas" or "I spilled oatmeal on my clo!" Not only do I not correct her, I just go along with it: "Yes, maybe we should get Daddy an exercise clo!"
This is going to come up on a spelling test in a couple years, and she's going to be really mad at me.