As for teen bodice patterns, unfortunately, there are none really out there specifically aimed at teens. (Which is very unfortunate and makes portraying a teen girl at events that much harder!) So, you'll either have to completely create your own through a toile or modify an existing pattern.
For a girl of 13 (if she is portraying 13, although if she looks older, she could portray an older age if she chose to), her skirts will be shorter, at least above the ankle. I would definitely go with short sleeves, as it's in the summer. A sheer cotton dress would be lovely and quite cool in the summer. A gathered bodice in the front with a back opening would be easy and very appropriate. Likewise, you can bone the bodice if you desire but you really don't have to. (Both of my summer dresses were never boned and they work just fine). You could do an off-the shoulder style, as well, but that's not necessary either (if you do an off-the shoulder, make sure she has plenty of sun screen on her shoulders, as they will get burnt!)
As for existing patterns, the Simplicity can work but are a hassle, and you'd have to completely redo the front placket to the back (which isn't too hard - you simply take out the overlap in the front, divide the bodice in the back into 2 pieces and then add 2" or so for a placket on either side). Making a custom toile is going to be your best bet, although to achieve a good fit she'll need a corset (Does she have one? That may be a good place to start). Practical Prinkery is a great book that will make the process way less frustrating, but I don't know how ordering it will work with the new baby.
Good luck on your adventure into sewing for a teen and I look forward to meeting this young lady next year!