This is Mo.
She's 11 (almost 12), 5'1.75" and 80-something pounds. She's an average size for her age. Her problem? Her legs are the same length as mine. I'm 5'6.5" and have about a 32 inch inseam. My legs are considered "regular."
Mo had basically given up on wearing jeans. Every single pair she tried on came up to her mid-calf. She could NOT find a pair that was long enough. She probably hadn't worn a pair for over 6 months.
The other day I realized at my lowest weight [107 lbs] I could fit a child's size 16. I was an adult 7, borderline 5. If a size 7 is equal to a girls' 16 then a size 10 in girls should roughly be the same as a 0.
I was basically right. I went in to Old Navy a few weeks ago. She is a size 00 in their Rockstar style jeans. They are super skinny and only a tiny bit too long. But, she loves them. They look and feel great.