How tall is she? Jeez, I'm an adult and only draw 25"! Anyway, considering she is likely to keep growing, I would probably build it for a bit more than 28".
Generally for a straight limbed, stiff handled bow you want 2x draw length plus 8-12" or so. I'd go for 68" personally and tiller it to 30", assuming it isn't taking too much set when yu get to 28"
Id shoot for 25#@28". Better to work on for, and figure draw weight later. Since your choice of weights leads me to believe that hunting isn't in the near future, I would go with the low end of your range.
If you decide to go to 30# I would stick with 1" but over that I would go wider, proportional to the weight. So maybe 1 1/4" for 35#.
Lastly, if your hickory is good, a backing shouldn't be necessary.