DDD - Get entity through aggregate, invoke method or hide entity method behind aggegrate method?

Consider the following scenario:

class Order:

    def __init__(self):
        self.lines = []

    def order(self, product_id, amount, price, discount=None):
        item = OrderItem(product_id, amount, price)
        self.lines.append(item)

class OrderItem:

    def __init__(self, product_id, amount, price, discount=None):
        self.product_id = product_id
        self.amount = amount
        self.price = price
        self.discount = discount or decimal.Decimal(0)

    def discount(self, amount):
        self.discount = discount

In the scenario that we want to add a discount to a specific line item, after the Order aggregate has been created, do I need to get the OrderItem entity from the aggregate, or expose a method on the aggregate?

E.g.

class Order:

    ...

    def discount_item(self, index, amount):
        self.lines[index].discount(amount)

And if I can invoke the OrderItem.discount() directly -- how do I keep track of the domain events published as a result of that?