Small women's vs children's backpacking packs

I've been introducing my girlfriend to backpacking but we're hit a road block in finding a pack that fits her well. She has a small frame at 5'0 and 85 lbs, and we have found that even small women's packs don't fit her size as well as children's packs. The best fit we've found is the Osprey Ace 50, but she has one complaint, which is that the bottom of the pack's frame uncomfortably presses on the top of her butt. Our not-so-farfetched theory is that a child's pack isn't designed with a woman's shape in mind, but we can't find a woman's pack that's small enough. It could also just be an adjustment problem, but i was hesitant to cinch up the shoulder straps too much and end up lifting the weight from her hips to her shoulders. Admittedly I'm not familiar with where a pack's hip strap should rest on a woman. Does anyone have some suggestions to help us out?

I am not quite as short as your girlfriend, but at 5'2", there are many women's packs that would be uncomfortable.  I have found that in-store fitting has worked for me every time I have purchased a pack.  The folks who fit packs for many people can check a selection of packs, whatever brand or designation (women's or children's), and give assistance in setting up and testing each pack in the store.

My general experience is that the hip straps should rest on the hips, not at the waist and not below the hips.  The pack should move with you, not slide down or ride up.  For someone with tiny hips, the question might become whether the hip straps can be tightened far enough to rest, rather than drop.

And the top of the pack should not be so high or tight into the head that you can't move your neck and head effectively.

I had a deuter kids bag for years. (Fox 30). It's quite good and adjustable for growing kids. The hip straps actually sat on me properly. I am.5'1". I would still be using it if my equally short sister didn't take it.

Thanks for the replies! She ended up finding a good fit with the Deuter Fox 40. 

I’ve been hunting for a pack as well. I’m 5’5” but 190lbs. My hips are very high and I have a tiny wasit. Hips bones are close to my waist. I get a dieter slim line and that doesn’t seem to work the greatest yet. They have a vari quick system I haven’t tried yet