I really like the end result, but I have a few notes on fit below.
This is size H on my very small 6 year old:


Here is my 11 year old, in a mix between size N/P/R (haha -- like the radio -- I knew I was raising a liberal!):


So, about the fit. I love the way the legs fit my very slender girlies. These are the first pants that have looked good on my 11 year old in ages! She is super long legged, so her legs were 4 sizes above her hip size. It's a good thing I sew!
The part I didn't like was the waist. On both girls, I chose the waist size according to their actual waist measurements, but when I sewed the pants, the waist was huge. Really huge. I ended up doing some seam ripping to take it in. The pattern has you cut elastic that is exactly the same measurement as the waist pieces, so you aren't stretching the elastic at all as you sew it down. This would work great to make a flat front elastic waist IF the waist pieces were just a tiny bit smaller than or even equal to your actual waist size. But these turned out much larger for my girls. Also, these didn't turn out as low rise as I expected them to be based on the description. So my suggestion is that you use a shorter piece of elastic and stretch the elastic as you sew it into the waistband.
For length, I chose the length based on their inseam measurement. This worked out perfect for my 11 year old (who is 5'3"). I might lengthen them even a little bit more for her so she has has growing room, but it was perfect the way it is. But for my 6 year old, I cut off about 3 inches before I hemmed them. They were WAY too long as drafted. Not a big deal, though, as that is an easy fix.
Also, the back waist piece is shorter than the front waist. So the side seam is farther back than I would expect. It seems like an odd choice that they made when drafting this pattern. It is not bad -- just not what I would have expected.
Overall, though, the end result is so good for my 11 year old that I will definitely sew some more of these.