Everyone has a different perspective about what is and is not "their business", but sometimes it helps to put yourself in the parent's place and respond in a way that is supportive and encouraging.
Sounds like a helpful approach! Mom might appreciate even a small gesture of kindness.
Put yourself in the parent's place - she might be coming home from a work, have a child who is sick and needs whatever she is buying, or have no one to stay with her kids while she shops. Keeping judgment out of the interaction is much better.
There's no easy answer to what to do in any situation. Some people are comfortable offering help to the parent, distracting the children, or just offering a smile and an "I know what it's like" look. To learn more about possible options, click here.