It think it’s important to plant this seed early, I would always joke about how the dogs are here “for a good time not for a long time” when my kids were growing up, it also encouraged them to live in the moment with our pets. Speak openly about the dog aging like you might to an adult peer, that way your child learns these scary thoughts are safe to have and you share them too so they can come to you for comfort. Obviously this is age appropriate advice little kids don’t need that last part.