Okay, my $0.02 here…
eBay may be your friend if you aren’t allergic to knuckleheads and can exercise a little patience. I have found a lot of great deals there, but then again price is relative to income and budget so you’ll have to make your own assessment there (obviously).
One thing to consider for packs would be looking for some from mass pack makers who offer comfortable backpacks in women’s XS. They’re usually well-featured and come in some pretty short torso sizes. Think: Gregory, Osprey, Deuter. It’ll be easier to find a used one from them than a cottage maker and the price may be more appealing, too. You can also keep an eye out for Osprey’s and Deuter’s youth packs on the cheap.
Another thing to consider, though probably still a little pricier, would be an adult’s half bag like climbers use. They’re usually well made and would fit like a long bag on a little dude. You can turn it inside out, slip an elastic head band over the lower X inches of the footbox, and then flip it right-side out to make it fit better. And remember, kids generally have a different tolerance for cold than us old folks, so a little flexibility in temp range may also be of benefit when shopping used gear.
And lastly, please don’t skimp on shoes or packs. Just like us, if they find an endeavor to be unpleasant they’re less likely to cotton to it. Shortcutting either of those may have unintended consequences for you if you hope to enjoy wilderness adventures with them for years to come. I’m 55 and my son is 22 and we’ve been backpacking together since his first trip when he was six years old. I bought him quality shoes and a women’s XS pack and he schlepped his own sleeping bag, pillow, and headlamp the whole weekend. Sixteen years later, we’re doing a week in the Yosemite backcountry this September.
I’m kinda jealous of you, even though I’ve already been there myself, because those were some amazing times. The enthusiasm, the adventure, the bonding… such a great phase in their lives and what a wonderful way to strengthen those tethers.