Playgrounds shouldn’t have chessboards if they also have rules prohibiting adults without kids. Stupid planning is responsible for this ridiculous outcome.
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.
If that IS the case, we have bigger problems.
With a simple rule change you wouldn’t even have to change anything about the playground layout. Just allow unaccompanied adults playing chess and nothing else and the problem is solved.