A young lady I know had a "Belle" princess dress, but it was rather too small for her. The sizing on these frocks seems rather skimpy, to me. There was a white underskirt, which had a plastic hoop threaded through the hem, like a crinoline.
I needed to make the dress larger in the chest and waist, lengthen it, and also change the underskirt. My friend also has to spend time in her wheelchair, and a rigid circle of plastic does not settle well, but pops up.
I began by removing the hoop and carefully cutting down the side seams and laying the dress out.
Next I cut panels to insert in each side [the front was brocade, but the back and bodice mostly just plain gold] and some white panels for the sides of the underskirt. I used golden tulle to make an extension frill, so that the underskirt would still give the dress some shape, but be easier to accommodate when sitting in the chair.
I was pleased with the final result. The dress has been altered to fit, with some growing room - but still looks remarkably similar.
Do you think Rosie will want Disney dresses when she is older? Liz and Steph never desired such things - but I did make other costumes for them when they needed them.