Children eating the box is a well known story from Urban Arrow riders. They somehow like the texture and when their sharp teeth are in, they can rip little parts off. And when there is a beginning, it's easier to proceed...
The material is EPP, a kind of Styrofoam. This makes our bikes more safe when there is an impact. Next to that, the material is really light in weight and durable in rain and sun. EPP is also easy to recycle. But please be careful your kid won't swallow the material. Just as dirt or other materials kids like to eat...
If you would like to repair your EPP box, you can use Sugru or another multi-purpose mouldable glue that made of a silicone rubber that can fix damages on your EPP box. There are a variety of colors on the market, including the black/grey of the EPP box.