Personally, first of all, I'd be getting rid of any scales. A 9 year old simply doesn't understand that she may weigh more than someone else, simply coz someone else is shorter than her.
My DD1 is 9. She is perfect. She's thin, but not skinny. A friend of hers is skinny & lanky. DD is always commenting on how she wishes she was that skinny. But right from the start I was truthful with her about it. I told her to look closer at the friend. Sure she's 'skinny', but look at the way her bones stick out. It doesn't look that nice does it? The friends own mother would love for her to put on a bit of weight, but I can't see it happening. She's like her dad.
I'd do everything to reassure her she's gorgeous the way she is, & keep an eye on her. Don't make a big deal out of it, you might make her think its something she needs to hide from you. Thats something you don't need.