I wish I knew for certain the answer to your question. I had decided to buy a Ladybug for my keychain to replace the kubotan that has been there for 16 years. I spent half an hour or more on the web, trying to find a plain-edged Ladybug. It was in vain. Everything I found was half serrated, or, I think in the case of the sheeps foot model, fully serrated.
I applaud your decision to get your boy involved with knives at a young age, and providing him some real responsibility. My Dad gave me a "jacknife" when I was 5. I've always been grateful for that.
I'm not as familiar with the smaller Spydercos as I would like to be. Frankly, I think that the majority of them have pretty wide -- top to bottom -- blades, that might be a bit much for a youngster to handle. I sure as heck would get him an FRN/Zytel handle, or even G-10, to maximize his grip on his knife.
I can see the wisdom in using a lock of some sort, but almost invariably, that entails a wider blade. You might consider the Russlock by Case. With the SS blade, it's not likely to get so crazy sharp cutting is inevitable. I saw the new green-handled bone one for about $37 the other day somewhere. That, of course, may have a longer blade than you'd like him to start out with.
The Kershaw Double Cross might be a very good knife, since it is slim like slipjoints, but has a liner lock. From my limited experience, Kershaw puts out extremely sharp knives. And, that knife has 2 blades, and is fairly expensive -- don't know your budget.
Off the top of my head, I think the two other knives I'd recommend would be the smaller CRKT LUS, and the Paragee by Outdoor Edge. Not Spydies of course. The LUS has a 2.5" blade, the Paragee about 3". The Paragee is pretty slim, and the LUS not too stocky, though the Paragee would likely fit a small hand better. My Paragee was extremely easy to cycle and unlock OOB. Much more so than most knives in that size or price range.
Only other thing would be a plain old slip joint. A Case Copperlock might work okay. Forgot them. They do have a lock, and a single blade.