Has your dog eaten small amounts of milk chocolate or chocolate-flavored biscuits?
Will It  harm my dog?
Better be cautious! 
Factors like Brand recipe, dog breed, Dog size and health condition of your dog, and amount of chocolate that your dog got a hold of, will  determine how your dog will respond will respond to chocolate ingestion. 

The Toxicity Calculator

Check the calculator to estimate if your dog is in danger. 
If the amount your pet ingested provides a dose that is potentially large enough to cause symptoms, seek veterinary help immediately. Call your veterinarian or the nearest emergency center for advice if you are in doubt or cannot tell how much chocolate your pet ate.

The best way to avoid accidents be proactive.
Secure all sweets in a place where your pet can not reach. Best places are top shelves, glass containers with secure cap, etc.

But the danger is not over. Some Sugar free human treats may include Xylitol as sweetener. 

Xylitol 

"In dogs, xylitol stimulates a rapid, dose-dependent insulin release that can result in profound hypoglycemia. Dosages of xylitol over ~75–100 mg/kg (34–45 mg/lb) have been associated with hypoglycemia in dogs. Some dogs ingesting xylitol at dosages >500 mg/kg (227 mg/lb) may develop severe hepatic insufficiency or failure." Read More