Kids eating products before getting to the till..A yay or a nay?

As some of you know that this is almost a habit for a lot of people...Is this right, should it be encouraged? Especially for mothers who seemingly have to do this to keep their children quiet or to pay attention. I have seen a lot of adults do it too with grapes, sweets and packs of crisps, biscuit and drinks...But do we all pay at the till? Because this is where my bone of contention lies..

What are your takes?