I modified the PosOrderLine model this way:

class PosOrderLine(models.Model):
    _inherit = "pos.order.line"
    child_products = fields.Many2many(comodel_name="product.product", 
                                relation="m2m_pos_testing_child_products", 
                                column1="m2m_id",
                                column2="product_id",
                                string="Child Products")

And I extended models.Orderline:

export_as_JSON: function () {
    var orderline = _super_orderline.export_as_JSON.call(this);
    orderline.child_products = [];
    //Loop through a Collection of models.Product
    this.child_products.each(_.bind(function (item) {
        //Python: [(6, None, item_ids)]
        return orderline.child_products.push([6, false, [item.id]]);
    }, this));
    return orderline;
},

This works fine when there's only one child product. But when I add multiple child products, only the first is saved.

How do I fix that?