So if you saw a sad child alone in the park you would not check? And you would discourage your own child from reaching out to anyone...even if they (the child) were in need?
For all we know the witness was some elderly woman walking her lapdogs who mentioned the sighting to an adult child, and that person reported it to police. Furthermore, if the witness has been publicly accused as being responsible for the lost child, all the more reason for her to be fearful of reporting the sighting.
A child alone at a park near a river in the Winter should be everyone's responsibility - not to the extent of calling police, but to at least ask the child if everything is all right.