As my young Padwan has indicated, to remain legal and avoid the paperwork for a tax stamp and a short barreled rifle, you need to be in excess of 16" of TOTAL barrel length.
Now, you can get an 11.5" rifled barrel, but you will have to have a 4.5" ( min ) muzzle break or mock suppressor that is permanently attached to the barrel.
There is no benefit to that, so you might as well get the full 16" barrel and get the most rifling out of the barrel you have.
With a GI profile barrel, one that is not pencil thin, you will get good accuracy out to 250 or 300 yards without too much trouble.
As for it "growing" with your girls, one of the benefits of the AR platform is that with two simple pins, you can change uppers have a totally new rifle. Long barrel varmint model, CQB Carbine, mid range target. All can be had without too much trouble.
What would you like the weapon to do? What is the expected accuracy? What would you like for the weapon to be capable of in the short and long term?
With these answers we can better help you build, or shop, for the best AR to suit your needs.