It was sweet and simple. It was very well written, but I would've changed some of the wording around to help the flow. For example,
"Happy Smile, Happy Smile.
I'll love it all the while."
It's still getting the point across that you love that smile, but you're not rhyming smile with smile.
"Special girl, Special girl.
To me, you mean the world."
You're still saying she's the world to you, but not throwing the flow off with extra words after the rhyming word. Sorry to be so nit picky, but I was throwing in my opinion. Nice job, I can see the radiating from this poem. Keep up the great work, and I'll keep reading.