I think in this case ‘concern troll’ means someone who can’t mind their own business and chooses to troll someone by stirring up unnecessary drama or conflict,

I think in this case ‘concern troll’ means someone who can’t mind their own business and chooses to troll someone by stirring up unnecessary drama or conflict, using insincere concern as a pretext. We know the concern is insincere because the OP’s example (children playing in their yard) cannot be considered concerning by a reasonable person.