That's right but "あなたの赤ちゃんはかわいいです。" is more smooth. "は" implies that not only your baby but also another babies are cute, I think. "が" implies that your baby is cuter than another babies, I think. Sorry to my poor English expression.

Yeah,It's right,you can say it

If you're looking at the baby, all you say is: 可愛い。 If the baby is not there then you say: お子さんは可愛いよ。