I wouldn't bother with a sign. I would definitely go out and confront them about the issue. Walking their dogs past your window is one thing but standing there for that long knowing the dogs upset your cats is pretty rude in my book. If you can hold on to your temper fine , if you can't then they've earned your wrath by their own rude behavior.
You could try getting a water gun and maybe have your window open wide enough to squirt the dogs in the face if possible ...lemon juice in the gun might work wonders. If the owners say anything just tell them you can't stand the noise the dogs make . Is there a landlord you could speak to about the issue?