I think Bowtech makes a bow that is completely upgradeable including draw length and draw weight. The draw length isn't too hard with today's bows just swap the module (for little to no cost to the pro shop) and you are done. The draw weight can be increased with new limbs and I think that is around $200, which is better than buying an entire new bow. Bowtech, Diamond, Hoyt, Reflex, PSE, and a couple of others companies make these expandable youth bows. I am more familiar with Mathews, but the only youth bow they have is the Gensis, which is good if you have 15 people shooting the same bow but I don't even recommend it for your situation. The Genesis is meant for target shooting and if I remember what you said you are hoping to get your kids into bowhunting soon. Now I will give my shameless advertising here look at Mission, I don't know whether they have an upgradeable type bow for youths, but I had a couple of buddies in Alaska that shot Mission, I even shot a few of them. Really nice bows, the wife almost got me one for the last Christmas but I have my heart set on a different bow, so I didn't get a new bow. Take a look online at the different manufactures and see what the offer. Honestly now days there are no POS bows on the market anymore, just differences in shootability comfort, silence when shooting them, etc. Best of luck and if they are hunting this fall I hope they both get one. Caution if they do manage to harvest this fall with their archery gear you may have fanatics on your hands.
WVBOWAK