Would bent sheet metal have any sharp corners or edges? For kids it's probably best to avoid those.

You can debur it as part of the manufacturing process, and then when it gets powder coating it smooths out even more. But you can just embed the sheet metal edges in a groove on a wooden side.

That depends on how much effort your spend on finish work. Files and sandpaper works wonders, but that take time. Or you can fold the edges over so that the sharp edges are on the inside (cars do this). The plywood is likely to cover all those sharp edges - plywood is easier to shape than sheet metal so this is a common cheat that can look good if done right.