Flea collars do contain toxic chemicals that are used to kill the fleas and ticks on the dog or cat but usually as long as the dog or cat does not try to eat the flea collar the flea collars are pretty safe for dogs and cats and are effective at keeping fleas off your animal.
Just don't allow your dog or cat to eat or chew on the flea collar and the dog or cat will be fine wearing the flea collar.