Well done to the OP. My God, would anyone seriously just walk past a sobbing lost child in a busy place?
Quite simple really, you just stay where you are as that would be your best guess as to the most likely place that the parent would find them. Then try to attract the attention of police or other appropriate person.
The kid in this case was also foreign. He would probably know to look for police in his own country, but might not recognise or be confident enough to do it in this country.
Anyway, if you put yourself in the position of that child, terrified and worrying, the OP stopped that and reunited him with his parents. Well done.