The supermarket chain came under fire from the Advertising Standards Authority, which told it to ensure its ads are “appropriately targeted in future”.
A video ad, shown during a series of children's nursery rhymes on YouTube, featured images of a number of bottles of spirits.
The ASA said: “We noted that both Morrisons and YouTube had processes in place that were intended to ensure that ads for alcohol were not directed at those under 18 years of age.
“However, we considered that the YouTube video in question was very unlikely to be viewed by an adult unless they were watching with a young child.
“We concluded that although Morrisons had taken all reasonable steps to ensure that the ad was targeted responsibly, it had not been targeted responsibly and therefore the ad breached the code.”
YouTube said it has mechanisms in place that should prevent alcohol content from being shown during family-friendly content, but did not recommend that users should log in with an account that declared they were over 18 years of age if they were watching YouTube with a minor.