The worst part of pet ownership is the fact that one day we will lose them. Whether they are young or old when they leave us, there is always a grieving process we must go through. Please read the following articles on how to handle the loss of a beloved pet.
This article describes the different developmental stages of children and how they grieve at each stage.
Some animals do have a grieving period after losing a housemate. This article describes the best way to accommodate them during their loss.
This article gives some insight on how to decide when it is time to euthanize your pet.