jd01,
Seriously, I recommend something EASY to open/close.. like a Kershaw Corral Creek II lockback. Stay away from difficult to open/close knives. Buy the knife yourself in person so you can try it out in-store.

Each child is different of course.. you'll have to be the judge of emotional/physical maturity & development. I've started off Many newbies into firearms/shooting, and there are Some where I've had to take the gun back and say "you're not ready yet"

When my son was 7 i bought him a Chipmunk bolt-action single-shot .22 LR. Within a week he was downing 2x4's at 20 yds.

Shortly I let him try a Swiss Army Classic iirc.. he Cut himself right away. James_bond is right, some SAKs are too tight, too small, blade cutouts too small, & blades too difficult to open/close for youngsters. He was a little de-moralized, but after he healed & some pep talk I found a Very-Easy-to open/close Kershaw Corral Creek II single-blade lockback. He could open/close it like a lil champ, and his confidence climbed with it.

So whatever make/price you choose.. I would highly recommend that it be EASY to open/close.

G'luck.. and Bless you!
our children, they grow so fast.. these years are Precious!
these rites/skills you induct them in will be with them for the rest of their lives.