This is mostly a place to hold several links to places that provide good information on selection. So you can skip this and scroll down to the bottom with the links and get the full picture. Go ahead, I won't be offended.
Still here? Okay, here's a few bits of my personal advice. If you are looking to buy, you're going to want to target 40-60 liter packs to have enough space for your gear. For smaller scouts, target the lower end of that range mostly for the torso size. Realize your scout will grow out of the pack before they finish scouting. And realize that the upper end of that range is marginal for multiday trips. You can fit everything in, but it comes with a cost. Literally, you'll pay more to get lightweight, compact gear to fit in your pack. If you continue with backpacking, go for it, because it will be more fun with less weight to carry.
Avoid getting a frameless pack. Frameless packs are fine as daypacks. The problem with a frameless pack is that the weight will be supported completely by your shoulders. The stiff frame is what allows the load to be partially shifted to your hips. External frame packs are typically better suited to on trail hiking, or at least places where there is not a lot of brush to catch on your gear. This would be most scout trips. Internal frame packs tend to have fewer external tie-down points which is the point; with fewer of those, there are less things sticking out to catch on brush as you bush-whack.
Features you may want to look for:
I have three backpacks. The first one I purchases was was a classic external frame pack, the Kelty Super Tioga 5600 (92 liters). It's the heaviest of the set, but you can tie things all over it and it's all but unbreakable. But it's nominally about 6 pounds of pack empty; I weighed mine at nearly 8 pounds, so I'm skeptical of that 6 pound specification. But this pack will let you carry stuff like a mule. Not that being a mule is any fun....
The second is a Kelty Red Cloud 90, a 90 liter pack. The hip belt stiffener broke and Kelty replaced the pack...for free. This weighs about 5 pounds. It has lots of compartments for organizing your gear.
The third is a ULA Circuit, a 68 liter pack with no compartments, everything goes into the bag, so I use ditty bags to organize my gear. It's the lightest of the bunch at a mere 3 pounds and has very nice pockets on the hip belt. It was also the most expensive. Weight down, price up.
For my sons, the first pack they had was a Kelty Tioga Jr external frame pack (34 liters). This worked for the first couple years of scouting at which point I purchased a Kelty Red Cloud 5000 ST (short torso). This is a 82 liter pack. The adjustments on this are enough they they are still using them.
Although I have an obvious history with Kelty, and they do make good packs, they are far from the only name in the game. Campmor, REI, EMS, all sell multiple brands and have staff to help with fitting and selection and they sometimes have sales. If you can borrow a pack from someone first, great. If you are comfortable ordering online, CampSaver.comalso has deals (where I got the Red Cloud packs), and never underestimate Amazon.comfor deals, too. As with anything, for some items, you're paying for brand name as much as for quality. Don't get me wrong, Arc'teryx makes great stuff but it's like buying a Lamborghini, you didn't just buy it because it's fast but because, hey, it's a Lamborghini.
Here are some links with good advice. These are all from REI, but they are not about buying REI gear specifically.