Disney has discontinued most of the old "official" Simplicity princess patterns. That Butterick is now the closest out there.
On the Belle dress, your result will be much better if you do the ruching on tapes, regardless of what the pattern suggests. This will also allow you to let the hem down easily to make the dress longer for awhile as the child grows (presuming that she doesn't get much larger around the rib cage.) I prefer to use cotton webbing tape with thin grosgrain ribbon underneath; in a somewhat contrasting color so that I can easily find it to let it down if needed. (I leave about a 10 inch tail at the waist seam on each of the ruching lines for that when ruching a child's dress.)
One of the best fabrics to use for comfortable princess dresses is cotton sateen, and the best cheap source is flat bedsheets. I get mine at Tuesday Morning.