Donuel,
She is doing nothing illegal nor does she plan to stop using a lawncare company. The local police official told you as much when he told you to pursue a civil suit.
The warning means that the daughter has discussed the matter with a lawyer and you are on notice to leave her mother alone.
Either take your complaint to court or drop it. The next step will be an Order of Protection which will make your life inconvenient at best.
Lawn companies generally treat a property once every six weeks. They do not always spray pesticides - often the treatment is simply lime or fertilizer. When you see the truck, close your windows and keep your child and pets inside for 4 hours and all will be well.
Or fight City Hall and lose.
Mary, a disinterested observer.