Bought when kid was 9.5yo about 15 months ago, took this pic 2 months ago and just had the raise the seat after this week's ride, not certain it'll make till the kid is 13. Went with a Cannondale xs 27.5 women's bike, the women's has a slight bend in the top tube that allowed for a little more standover which was very necessary when we bought it. Tried trek and giant and the kid couldn't clear the top tubes flat footed on their smallest 27.5" mountain bikes. My soon to be 13yo is on a size small cannondale f400 like the one someone else posted and she towers on it at 5'1" (she's also short for her age) and it will need to be replaced this year though really I'm gonna make her squeeze out the summer on it and pay or something much nicer. So I'd shoot for a 27.5" wheel for the better ride quality and see if a small couldn't be squeezed but go with an XS if you have to. Better to have them enjoy riding with you and replace it sooner than get too big and turn them off of riding.