I was looking for my 10 yo sons XL but couldnt find it just now, my thoughts are that Nintendo must have taken in account small hands while designing the XL and your daughter will get used to it anyway.
I have a DSi and a DSi xL. when my 4 year old niece comes over her and her 5 yr old brother pick up the xL first. Whoever gets there first gets the bigger one. They are definitely beginners but I think that is why they prefer the bigger screen. they do a lot of smartkidss/diego/disney princess/my little pony gaming plus they love to take pictures and then draw on them, so do my 7 and 9 yr old niece.