But why make her 5 years old? That just makes MC either a pedo or it confirms that mc and wmc will not become a couple
The Japanese, at least the ones that write fantasy manga, have no idea how to handle romance. Specifically, being in an adult sexual relationship.
To avoid that pitfall but still provide lonely readers fanservice, they make situations where that type of relationship is very unlikely. Either with super dense protagonists, socially crippled loners, inhuman beings such as your slimes, your skeletons, your swords, your vending machines…
And yes, putting an adult person in a child’s body, or in this case vice versa.