It really makes no sense. After all, it was a shoe store and I'm sure lots of people enter and leave--what difference would it make if an employee's fiance brought a child in through the front door, as long as they (the fiance and the child) behaved, and weren't acting disruptive to customers?
It doesn't make sense.
If there were some minutes left until the end of the aunt's shift, Ross could have taken the boy through the front door, to the back of the store, to wait until aunt was off work.
Apparently Ross was leaving the boy in his aunt's/foster mom's care because he had to go to a class. That's the story, anyway.