At about that age I bought several lower cost knives, including a simple SAK and several locking blade knives (yeah, my wife was thrilled), and let my kids use each one. The one they preferred was the Gerber LST II for about $19. It was smallish (2.56" blade), had a locking blade, and they found it easy enough to open with one hand (which is sometimes tough for a kid). It does not have a clip, which is fine with me as I feared the younger kids would have lost it when clipped.

Now my son, who will be 12 years old in two months and in Boy Socuts, but taller than his mother and wearing size 13 shoes, is carrying a Kabar "Folding Spear", which has a bit longer blade (3") and a clip, and had a Victorinox StayGlow Yellow Climber from knifecenter.com in his daypack. He very recently completed his First Class rank and so I gave him a Leatherman Wave as a long promised reward.

My daughter, who is 10, still much prefers the Gerber LST II to the Kabar Folding Spear that I also bought her, and also carries a Victorinox StayGlow Yellow Climber in her daypack.