Oh… wow. You really can't blame yourself like this. You did more than a lot of people would have done – you advised a course of action to the clerk and waited to see security come to help. I don't know what any of us might have done in that situation. But, you couldn't have taken the little girl at her request.

Still, I understand why you feel this way. We want to do more even knowing we have no power to do so. If you'd stayed further or imposed yourself into the situation, you would have become a different problem and a distraction from whatever help security was offering the woman and child.