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.

Ummm... better hope the ml isn't the duke or something