Congrats on getting the family into the sport! The suggestion on using the 2 blade head was a good one. There's nothing wrong with the killing power of a 2 blade head; in fact I'd bet a nickel that more game has been killed with a 2 blade head than a 3 blade head by a factor of 2 and maybe 3 (not counting stone heads of way back). The reasoning behind the 2 blade suggestion is based on physics and is sound: the mechanical advantage of a 2 blade head is significantly greater than that of a 3 or 4 blade head. With equal shot placement the 2 blade will penetrate better in every situation up to the point where both heads create a pass through. At 40 pounds, a pass through is not a guarantee, so the penetration gained is worth losing the extra blade worth of cutting. At 70 pounds good shot placement will give a good pass through with the biggest of animals, so the extra cutting is a nice luxury. Keep the arrows heavy and use a good quality head that you can keep sharp, and she'll be just fine for deer or even elk with good placement of the shot. Magnus Stingers can't be beat for your description, plus they have a lifetime guarantee, so you'll only have to buy so many of them when she starts practicing with them and inevitably hitting rocks on misses or pass throughs.
Keep her practicing, get her close, and you only carry a video camera when she's hunting because you'll want those memories on film to re-live for a lifetime. Good luck!