Perhaps I can offer my observations and save you some money. As young as your boys are, I wouldn't get too big a volume pack. For lightweight but volumous items, you can use a compression bag. And assuming you go together, you can adjust their packweights as needed by loading yours with the compact heavier items.
Three years ago I bought my 2 boys (10 and 12 at the time), my wife and myself new packs. I got the boys each a REI UL30 pack (30 liter, 2# 7 oz, 15-17" torso, recommended carrying weight 24-28#) and a REI UL45 (45 liter, 2# 12 oz) for my wife and myself. We have camped since then, but I am the only one that has used my pack, backpacking or otherwise. The other three packs still have tags on them. None of my family seems to be into it, so I've gone without them.
I have been very happy with my UL45 pack. When I bought that pack, an employee expressed the idea to me that he wished he had the smaller REI UL30 (30 liter). I expressed surprise that it would hold his necessary gear.
I was reading on BPL prior to that time, and continue to do so daily. So, of course, my equipment has lightened and takes up less volume. I've been testing various pack volumes for different length and time of year trips.
Today I wish one of my UL30's was a medium so that I could use it myself. However, unfortunately, they are both smalls. Also today, among my other packs, I have an Ion which is about 24 liters. I haven't yet tested it to determine whether the size is sufficient for my needs.
It may be a long shot to get packs that work throughout all their teenage years. Then again, it may work out well for you with your choices. I just know that my own evolution has gone through many phases of trying out and lightening up.