The Bible tells us that Jesus's message is so easy a little child can understand. This song is just that way. It's lyrics and music so simple and beautiful. It's the first Christmas carol I ever learned. Now my daughter loves to sing it too.
The last line is often rendered, "And take us to Heaven to live with thee there."
I prefer "fit us for Heaven" because I think that's exactly what Jesus does with us while we're living on this earth. He shapes us and molds us, making us fit to live with him in Heaven.
I hope you've all had a wonderful Christmas with many reminders of the baby in the manger...the man he became...and the Savior he is today.