Okay, specifically to the OP we are talking about a 2 year old. A 2 year old who is walking and only 1-2 feet away from his mother. He has not run amok. He has not caused an accident. He has not bumped into a person or a cart.
He has drifted, yes, but his mother has told him to "watch out" and thus remedied the situation.
To me, the key is the unspoken word "yet". Just because he has not done so as of now does not mean that he will not do so in the future. Calling out to the toddler is not nearly as safe as having physical control over him.