Why? She isn't actually a child, only 17 and he turned 20. Adulthood is this world is most likely a different age compared to our standards. I have read stories where 16 was considered an adult and people complained fls are a minor marrying older mls. This one could be the same premise. And she is only under a spell that rewinds time so she is physically a child but not really a child.
Even though it maybe be legal in this type of stories it's just weird y'know?? （￣へ￣）
But we still have to remember, its still stories. As well, things like this happened before centuries ago when marrying young was needed to be done because of the life expectancy was low.
Ummm... better hope the ml isn't the duke or something