The way I read the OP's post, she got there and the store would not let her have access to her son. Based on 30 years experience in the practice of law, my opinion is that the store's conduct in refusing to allow the mother access to her son can be justified by any reasonable interpretation of the immunity statute you quoted.

Damages are, of course, an entirely different matter and I do not suggest that filing a lawsuit should necessarily be done. It does appear that the store exceeded the bounds and she should consult a local attorney.