Questions About Pet Loss

Your veterinarian will give your pet an intravenous injection, which is generally a combination of chemicals designed to give a quick and painless termination of nerve and muscle functions. Many vets will also give the pet a sedative first to relax them. The American Humane Association offers a good explanation as well: Euthanasia Decision
There is no simple answer since it will depend on their age, maturity level and life experience. KidsHealth offers very good advice: When a Pet Dies