I just wanted to share my experiences making this!
I'd call myself an intermediate sewist - I make my daughter a few dresses a year. I was intimidated by the 3 scissors out of 4 rating, but ultimately it was appropriate for someone with some previous sewing experience (as the rating descriptors say). Adding a zipper wasn't nearly as scary as I expected!
I picked size 5 for my almost 4 year old daughter based primarily on her height (to ensure that the torso and skirt were long enough). I was afraid it might be too wide around for her after measuring, but the bodice muslin fit perfectly without adjustments. For reference, she is around 44" tall and about 41lb and wears a size 5 in most brands of clothing.
The one mistake I realized I made after sewing is that I put the skirt lining in backwards, with the tulle against the wearer. I have to turn that around for comfort but otherwise everything worked out really well!
She wanted it in the model fabric, so we used that + a coordinating blue.
