Agreed, seems like the kid didn't age (unless she was taken as a baby) but the photos, unless those were the "age progressed" ones, imply she went missing at around age 8 and is STILL around age 8. Stunted growth from captivity? Her deplorable condition (undead?) would certainly warrant an extended hospital stay.
I suppose for the horror element they can handwave these issues but that's already straining disbelief. Though maybe this film gets so out there that this notion of taking the kid home is the least of it.