"You are very precious to me"means"你对我来说非常珍贵“or”我很珍惜你，你对我来说很重要“in Chinese.

你可以说：你是我的一切。Nǐ shì wǒ de yí qiè.

Yes, "You are very precious to me" is good idiomatic English. I am not sure a child younger than seven or eight years old would know the word "precious" but of course they will learn it at any age if you are hugging them when you say it!

You are very precious to me. Here are some sentences plus literal translation. 0-5 ： 你真是我的好宝贝！(You are really a good baby for me.) 6-12：你是个好孩子，我非常喜欢你！(Good boy/girl, I like you very much.) 13-16：你很棒，我非常珍惜我们之间的友谊！(You are really excellent, and I do cherish our friendship.)

Thank you, everyone, for your answers!