Back in a former life, when I used to teach skiing, and as a part of that, had to take my turn teaching young kids, I learned that age is a very crude way to think about the muscle and coordination of children. The question isn't at what age should a kid get their first knife, the question is do they have the hand strength and coordination to close it safely?
I started my kids with Opinels and the Vic Classic. The Opinel lacks a spring and because of this, is safer to close. The Vic Classic is so small and the spring so weak (a good thing) that I think it's inside the range of safety. Both of these knives lack the "cool factor" that larger SAKs have.
If you go with a larger SAK, my recommendation would be to watch and monitor to ensure they can safely close the knife without loosing control of the blade's snap.
Another option you might consider is the Leatherman Leap, which is designed specifically for young kids. It features a removable knife blade, so you can add the blade or take it out depending on the situation. If you have a kid who caaaaan't quite manage the knife blade safely without being monitored, this gives you the option of putting it in only when you can monitor their use and then take it out so they can safely play around with other tools, which helps them develop the coordination they need.