Remember that the Raanic laws governing slavery was set right after the severance, by a society that was still mostly civil. In reality, little remains of those laws today, two chaotic centuries later. Even if it's still technically prohibited to kill or severely beat your slave for no apparent reason, it's something most of your peers would just shrug about, and no law enforcer would ever act on it unless it's done in the open on a civilized street. And even then you'd only face a monetary fine.
Raanic society has not banned underaged slavery (including for sexual use), as one can read in some of the girls' backgrounds. Taking a very young child for such purposes would be frowned upon by all instances but it's not uncommon or disapproved, from the society's viewpoint, to use younger teenagers in the "full spectrum" of slavery.
Note that this is strictly lore-wise and not something that the player could, or ever will be able to participate in. Having excessive gore and underaged content would force MoR underground and violate even the (very) lenient rules of F95. Any screenshots stating otherwise are either $age variable cheats or mods that've replaced images.