Tesoro Compadre, White's Coinmaster, Ace 150/250 (personally I can't stand Garrett tones), Fisher F2 are probably the top name brand entry level detectors. She'll also need a good trowel...a good quality garden trowel or one from White's or Garrett's. Or if you live near a beach a sand scoop. A pin pointer is a must for everywhere but beach hunting.. A Cen Tech pinpointer is less than $20 and well worth the money.
Used machines can also be a good choice.
Before her B-day she can practice cutting a flap and replacing it.
Where will she be able to detect? Of course your yard will be the first place to hunt. But that may play into a machine decision. She should spend time on here learning lots of good stuff. One thing she absolutely needs to take to heart is that good targets won't just jump out of the ground. Plus junk is part of the game. She'll have to dig tons of pull tabs if she wants to find gold. (And she will at some point.)