If you have the Romantic Garden pack you can use the wishing well to ask for immortality, it'll give you ingredients to make ambrosia OR if you have GTW you can use the serum table to make a serum to bring them back to life I believe.
Otherwise, if you've only base game I think your only options are to max out Cooking or Writing > Gather ingredients to cook Ambrosia or spend many days writing a Book of Life. There's a guide somewhere on how to get the ingredients though and you can use cheats to max their skill out
