This exact thing happened to me, and it was so annoying! I search high and low all over the web and forums for an answer, and finally i found a solution. Here is what I did.
- Turn on cheats and find the dead dogs ID numbers using this cheat: sims.get_sim_id_by_name [PlayedPetFirstName] [PlayedPetLastName]. Get each one individually and make sure you copy the number down so you don't forget it.
- summon the dog to your house by using this cheat: sims.summon_sim_to_zone simID
- once both the ghosts of the dogs are at the lot, add them to your family (shift click on them, press add to family)
- Then you can take them to the vets, buy ambrosia treats for both of them and bring them back to life
- they will come back to life as elders, so will live in that stage for a bit, and eventually die of old age, leaving you with the urn/gravestone.
If you want them to die immediately then you can find cheats to kill them: traits.equip_trait Ghost_OldAge [Pet ID]
I hope this helped you! It really helped me.