Technical details at the link are pretty slim. "Polyamide fiber" = nylon.

There are a couple of ways you could do it. Some plastic parts are made sort of like I-beams of fiberglass reinforced plastic. That seems like it would be a natural for kids bikes. It's pretty easy to imagine something like cafeteria trays, too. You could also probably make a bike frame about the same way they do big plastic kiddie toys like Cozy Coupes, in which plastic is rolled around in a mold until the whole inside surface of the mold is coated.