Earlier quoted context omitted.
Not to out myself as a nerd, but if my history is any indication, some kids play chess. And even checkers.
Sure, but how is that relevant for my argument? Kids and adults enjoy chess and checkers, both should have access to public chessboards, slides and swings are for the most part only enjoyed by and only safe for kids.
Amending the sign to read something like "no adults, unless playing or watching chess, but not if a child has been waiting to play, except ..." would get complicated fast. Just go to a different park already.