But how are you going to enforce that liability? Making and selling knock-off "Lego ™" is already "illegal"¹ yet Ali-express is filled with this. How would this change when this knock-off-lego is also made with poisonous plastics?

Point is that e.g. within the EU the liability is clear and enforcable and the manufacturer has a role there. But with imported products, it doesn't. That liability lies entirely on the importer.

¹ (in legislations that recognize the International Trademark and copyright laws)