I love Barbara Walker's Knitting from the Top. One of my favorite knitbloggers, The Boy Who Knits, was having issues with the simultaneous set-in sleeve, so it sparked my motivation to share my tweaks & changes with y'all. In no way am I criticizing the great Ms. W. These changes just help things drape better on the bodies I knit for, so maybe they'll work for yours, too.
If you don't have the book, the following will probably make no sense to you whatsoever.
If the shoulders are really broad, a neck opening that 1/3 of the shoulder measurement is fine. For normal or narrow shoulders, especially kids, try 1/2 for the neck, then distribute the remaining, 1/4 on each shoulder.
While working down from the shoulders, instead of stopping when the edge is 1/3, narrow this down to 1/4 of the sleeve measurement. After picking-up the shoulder sts, increase the sleeves EVERY ROW until 1 inch worth of sts are added on each sleeve, using the lifted-strand-make-1. Then return to increasing sleeves every OTHER row, or whatever rate of increase works best for the stitch pattern.
Increase each sleeve, until minus the number of sts to be added in 1 inch of rows when increasing every other row, and minus the extra cast-on stitches needed for the body. If you reach the correct number of sleeve stitches before reaching armhole depth, knit even until the armhole depth is reached. If you reach armhole depth before obtaining the correct number of sleeve sts, cast-on extra at the underarm, and decrease them away in the body.
Next, increase both the sleeve and body stitches for 1 inch of rows, or 8 extra stitches every other round. Then, the underarm stitches are cast on, the sleeves are put on holders, and continue working the body.
When it's time to work the sleeves, pick up 2 extra stitches, one at each edge of the underarm cast-on, to help close those offending holes that often occur. Then, decrease those extra stitches away on the following round.
I hope this helps you, Kenny!