I understand that. If someone called the police and said they spotted a child wearing corduroys or with a chocolate milk mustache, would the police be obliged to investigate?
I said the standard for investigation is lower than the standard for determination ... I didn’t say there is NO standard, or virtually no standard, as is the case in the examples you cited.
Hence my gripe here is not with the police, but with the onlookers who I think should stop acting like the whole world is a NYC subway where you can’t talk to or even make eye contact with the people around you.
Absent any actions that would cause a reasonable person to fear retaliation, they should have first approached the family and said - pardon me, I’m concerned about your child, is everything ok? - then, after that, if they still had concerns (which they likely would not, in this case), they could have made a more informed decision about calling the police.