Changing Neck Line

Hi, I made matching BB dresses for my two older girls and I think they turned out great! My 6 year old loves hers, but my 8 year old complains that the neckline is too tight, especially when she sits down, because it pulls back on her and she feels like it's choking her. So, she leaves the top button unbuttoned...at least she has long hair to cover it! :-) I did adjust the bodice length on hers (my older girls are tall and thin), and I'm afraid I made it longer than I should have, so I've wondered if that is impacting how it pulls when she sits, but I think she'd just prefer a looser neckline. Or maybe I made the bodice too tight? But it seems to have just enough give, although she complains it pulls her shoulders back. She is still young enough that she has a hard time finding the technical vocabulary to describe her complaints; it took a good two weeks to figure out her vision for her Halloween costume, ha ha. Okay, so here's my question. I'm wanting to make matching Christmas dresses for all four of my girls but I want to make sure my oldest daughter is comfortable. I have a great quilting cotton weight buffalo plaid, and I planned to make the BB dress silhouette with the Peter Pan collar. Where should I start with adjusting the bodice to make her more comfortable, and how should I adjust the collar once I've adjusted the bodice? Or would I be better off skipping the collar? The thing that's making me nervous is figuring out the collar and bodice adjustments together. But maybe the collar doesn't need adjustment?