This sounds like quite an inrresponsible mother having a fit so she would appear to be concerned.
1 - Neither the kid nor you saw her in the vacinity.
2 - She was far enough away from her child that she was not aware of the crying.
3 - she was so inattentive that you were able to walk the child to the security desk before she noticed anything wrong.
4 - she completely disregarded that you were at a security desk; a very good sign that you are NOT abducting the child.
Not everyone who has children is qualified to raise them.
And you would have been perfectly justified in slapping her back - but that is just my opinion, not the law's.