Actually, it's not.
Depending on where the fracture is, even a hard cast isn't completely protective of an injury. It is relatively protective.
This is why frequently forearm fractures may be casted above the elbow for several weeks until a callous forms and they are stable to the simple movement of the elbow and shortening of the cast to below the elbow (but not stable to a hard fall onto the cast/arm).
I think it is ludicrous to have your kid play with a hard cast, purely for the issue of additional (and unnecessary) injury risk
As I coach, I wouldn't allow it (but fortunately the league had very clear rules on this).
I've seen a few kids play with various iterations of padded forearms, and every single time the cast has been used as a blunt object against other players--- intentionally or not I do not know