"She was so patient with him and she made him feel like a million bucks," said Kimberly Grandinette from Ohio, America.
"He skipped out of the store with a smile on his face."
Grandinette's baby son was in hospital and the mum-of-two was worried about the effect this was having on her elder son, who was "having a pretty rough time with all of the changes in routine and changes at home."
Grandinette took her three-year-old with her to the Meijer supermarket, where she got into conversation with cashier Kristen Clark.
The mum started to tell Clark about what was going on with her family, when mid-conversation her son piped up and asked Clark if he could help her ring up the groceries.
Without hestiating, Clark said "certainly" and allowed him behind the counter.
Grandinette snapped a photo of the touching moment and shared it on the supermarket chain's Facebook page.
"Thank you, Kristen, for making my son's day (and my day) a bit brighter," she wrote.
The post has gone viral with more than 140,000 likes, 11,000 shares and nearly 4,000 comments within three days of being posted on Sunday 13 December.
Clark's parents were among the first to comment on the post.
“That's my daughter. I'm so proud of her. #prouddaddy,” wrote Carl Clark.
Diane Clark added, "That's my wonderful daughter. I'm so thankful for her taking the time to make his day because that's how she is, and I'm thankful for you to notice.
"Happy Holidays to you and your family."