I don't doubt that the Walmart lawyer will attempt to argue that punishing Walmart for doing this may endanger children at a later date, since employees will be afraid to follow up on concerns.
To my mind, the people at fault were the Police, Walmart are free to raise concerns about photos, but the Police reaction of 'Guilty until proven Innocent' was wholly wrong, a simple phsychological evaluation in a few hours would have provided enough evidence to warrant leaving the children with their parents whilst the investigation took place, but instead they assumed the children were being abused and took them away from their family, therefore creating their own damage to both the children and the parents. It was a stupid, paranoid reaction from the Police that when wholly against the basic tenets of law, as well as causing irrepairable harm to a family.
Personally, I'd be looking to sue the child protection and law enforcement side of the argument rather than the group that raised the concern, since they had a far better picture of the family environment these kids lived in than Walmart does.