Here at Goober Pea Designs, we recognize that kids come in all different shapes and sizes, and we strongly encourage blending sizes to get the perfect fit! My own kids typically fall into different sizes for height and width, so I've had plenty of experience testing these techniques and figuring out the easiest and/or most accurate ways to do it.
There are two main methods that I usually recommend. In this post I'm going to cover the standard method that I recommend. This is the best method to used when blending a smaller chest with a bigger height (since it adds height proportionately, instead of just at the bottom). It also works well when blending 1-2 sizes difference for a smaller height and larger chest. For 3 sizes or more difference, the armscye and sleeve cap can get a little too distorted, and it's best to use the shortening method instead (or alternatively, blend 1-2 sizes using the standard method and shorten the rest of the extra length).
IMPORTANT: This method only works with patterns that are nested at the bottom of the armscye. All Goober Pea Designs patterns are nested this way.
Compare the child's measurements to the size chart and determine their CHEST SIZE and HEIGHT SIZE. If both sizes are the same, no blending is necessary. If they are different, print both sizes.
The general principal of this blending method is that all HORIZONTAL lines are the HEIGHT SIZE, and all VERTICAL lines are the CHEST SIZE. Then you just extend those lines to meet each other. There are a couple of curved lines that are kind of both horizontal and vertical though - the neckline and the armscye. The NECKLINE is always the height size. The ARMSCYE is always the chest size, except at the very top where it is shortened or extended to meet the horizontal shoulder line (which is the height size).
So to sum up:
Similar to the bodices, the vertical lines (sides) of the sleeves use the CHEST SIZE, and the horizontal lines (hem) use the HEIGHT SIZE.
The sleeve cap is a blend of both - using the height of the HEIGHT SIZE at the top of the sleeve cap, and then gradually blending to the CHEST SIZE at the sides.
Easy right? Ok, it may seem a little complicated at first, but trust me, when you get used to doing this it will become second nature, especially if you have kids that need it all the time.
If you need a little extra guidance, here is a video I made of this process: (closed captions will be added soon, but are not currently available)