I gave my young grandson a Holga and also an older manual focus SLR. He got about the same hit-rate of images regarding focus. With the Holga, the simple zone focus method using graphic symbols on the lens was just as efficient as expecting him to carefully focus through the SLR viewfinder, which he'd oftentimes forget to do. Exposure control is problematic with either camera. For a youngster, I'd recommend auto-everything and at a price point where you don't mind if it gets lost or broken.
Really, with film cameras, the biggest expense with kids will be paying the minilab for prints of images most of which are blurry or blown out/too dark. In this regard a cheap digital might be a better learning tool for the budding photographer. But it also depends on subject matter. Him taking photos of his kid friends might require a simple digital, simply due to the quick-moving nature of the action shots, but if it's still life images (toy soldiers, etc.), and the kid displays a bit of patience, then I could see an old press camera or even pinhole with LF paper negatives (or Harman DPP) might do the trick.
~Joe