You can say she is beautiful, pretty, sweet, cute, kind, smart or grown up.
For example, "you are very pretty" or "you are very smart".
You can also compliment her clothes, "I like your dress" or "your shoes are very pretty".
Most children like to be called grown up or smart so you can say, "that is very clever of you to do that by yourself" or "that is very grown up of you to say that".