The man, Keisuke, lost his wife, Takae, ten years ago. Then, one day, an elementary school student named Marika shows up and claims to be the reincarnation of his wife.¹
Pedophilia-Lite
Keisuke takes a picture of the reincarnated Takae (who is ten years old) and she asks what he would say if the authorities found out. His answer? That it was a picture of his wife. While the show, overall, doesn't seem super "perverted," especially considering what it could have turned into, it is still weird and wrong to call a little child your wife, even if it's a reincarnation.
Possession
When Takae died, her spirit later takes over another girl named Marika.² This is not Christian for two reasons. First, the Bible teaches that when one dies, their soul either goes to be with the Lord (2 Corinthians 5:6-8) or to Hell (Luke 16:22-23).
Secondly, this anime teaches that the human soul, when separated from the body, is basically the same as a devil. Devils can possess people, and that's pretty much what happens here as well.
Conclusion
Even if you want to claim this anime is fine, it has a terrible, non-biblical view of the afterlife. It also has minor hints at pedophilia, which is never okay. The Bible says in 1 Thessalonians 5:22 to "Abstain from all appearance of evil." At the very least, the title and premise of the show disqualify it from being okay to watch.
Sources:
1. "TsumaSho: If My Wife Becomes an Elementary School Student.: Parents guide." IMDb, imdb.com/title/tt32832434/parentalguide. Accessed 29 Jan. 2025.
2. "Wife, Become an Elementary School Student." Namuwiki, en.namu.wiki/w/아내%2C 초등학생이 되다. Accessed 29 Jan. 2025.