It is extremely dangerous to give a dog any medication without consulting a veterinarian. Even a small amount of Valium (diazepam) can be toxic to dogs. The correct dosage for a dog is dependent on their weight, age, and individual health conditions.
Please contact your veterinarian or the ASPCA Animal Poison Control Center immediately if you suspect your dog has ingested Valium or any other medication.
Here is the ASPCA Animal Poison Control Center's phone number: (888) 426-4435
They can provide you with immediate guidance on what to do.