You're right, a metal one would last longer, and made of something like titanium it wouldn't even rust. But for a small child developing, the goal is to brace

You're right, a metal one would last longer, and made of something like titanium it wouldn't even rust. But for a small child developing, the goal is to brace it long enough for the child's body to grow out of it, and then it's reabsorbed. That way the body can use it as supportive scaffolding, but eventually heals away. Which isn't as snazzy as a cyborg kid, but pretty cool that after a few years the child is running on their own support!