Okay, asking the hard questions here. If succubus existed IRL, and there were little succubus girls, how would the law deal with their, uh, feeding habits?
Legal jurisdictions around the world will be split between two factions:
Those who approach the matter from a scientific angle, and upon obtaining incontrovertible proof that their feeding needs are as biologically hardwired and essential as the need for water, oxygen and various nutrients is in humans, they will (often reluctantly) acquiesce to this newly discovered reality and attempt to adjust their moral worldviews and laws to accomodate it.
Moral hardliners who refuse to accept the very idea that such a diet is even remotely necessary and will sanction all sorts of "conversion therapy" to "cure" the "obviously corrupted" child succubi/incubi, then constantly cover up all the horrendous results of their irrational methodology' repeated failure while jumping through one logical hoop after another to avoid confronting the truth that said failure is screaming in their faces.
In my opinion, if succubi/incubi do exist irl, the age restrictions on marriage might get lifted to solve the problem legally. But depends on how the previous generations sees those races, either they are subjugated for causing mortal deaths or treated equally. If the former, then the chance of them to continue to exist are low unless the government decided to conserve the endagered species for whatever reasons. But if the latter then the law as suggested earlier might have been implemented ages ago to let the humans and demons coexists.
Also it depends on the biology of the succubus. Young succubi/incubus may not have the need of life forces yet until they come of age, they might only be more mischievious as shown in this manga. So they can be protected under the law. However, if they do need life forces even during their childhood, then i guess they have no choice but to let them quench their desires legally through young marriage.
Whatever the circumstances might happen, the unit to control succubus behaviour would have to be made as succubus are still considered a threat to human's lifespan, in concern of human's extinction in the long run.
Don't mean to sound negative or anything but imagine the world is controlled by succubi/incubus, doesnt that make the humans their livestock? Meh i'm tired, thinking about solving problems hurts my head. Humans don't even have the perfect solution to prevent disputes among themselves.