Grammie - I'm so sorry for this problem. I haven't been here recently, so am just seeing this. The apron is adorable and I know you are sick that it doesn't fit. I will have to pick up the original one from my daughter and check the measurements to see if the magazine changed anything. I do have the test sample that I made prior to making the ones for the magazine, and the size 2 had a neck opening that opens to 20". The average head measurement for a 2 year old is 17". I know that all 3 of our granddaughter's wore the dresses that were in the magazine without any problems. One of them has a much larger head than the other 2.
I did see on the pattern pull out that they indicate that the Chantilly version uses a 3/8" seam allowance while the Under the Sea the neckline is a bound edge. That is rather confusing to me. I also see that they didn't adjust the placket for larger sizes, which should have been done. Kids heads grow. Obviously, if you're turning a bias band to the inside, as with the Chantilly version, the neckline will be larger than if you use the edge as the finished edge. I think that they forgot to mention that you need to trim away the seam allowance for the Under the Sea version. Ugh!
I wish I could think of a quick and easy solution for the problem, but one escapes me at the moment. I think you would have to remove the bias at the back neck and then make a longer placket. I'm so sorry. I sure with the magazine would have sent the designers the pattern/instructions for review prior to publishing, but that never happened. So many problems could have been avoided that way.
Just a FYI - if you ever make the version with the lace, they put the wrong measurement for that as well. You need twice the amount of lace indicated to go around the skirt portion. Ugh!