The best strategy for having at least one child who survives their adolescence is to have lots of them. Lavinia rejects this strategy before it can even suggest itself. She made it through the Scholomance alive, but neither of her younger brothers did. Lavinia doesn't have an enclave; doesn't have a clan bigger than herself and her parents. She is willing to make the gamble once. She starts preparing for years beforehand, writing up everything she can remember about how to survive in the halls of the school, writing up lists of the most advantageous per unit weight potions she can brew, taking high-paying mundane jobs she hates to build mana and money and living without spending a drop more of either than she has to. Thirty well-made crystals full of mana will only buy you another quarter-kilo of weight allowance, but a quarter-kilo of valuable trade goods could save her child's life. 

She makes it clear to her parents that while she forgives them for choosing the strategy they did--can't blame them, really; they only had three children, lots of enclaveless wizard couples have lots more--if they want a grandchild, if they want to be part of that grandchild's life, they can get in line with her plan. They agree, which is good; even if she hadn't forgiven them, three people building up mana and other resources is better than one. 

It would be better to have more than three. Lavinia looks for a husband. She doesn't intend to marry for love; love is a lovely idea, but she is saving all of hers for her child. If she happened to fall in love anyway, that would be nice. 

It doesn't happen. Lavinia isn't surprised. She isn't really the falling-in-love type; she's had flings, and enjoyed them, but love songs and gooey looks have always struck her as faintly dull.

She rejects three promising prospects with more relatives, more power, more resources than she has because they make it clear to her that they aren't going to be satisfied with only one child. Tripling the available resources isn't a good deal if she has to divide them again by a quarter. 

 

She doesn't find another wizard who's onboard with her plan to have exactly one child and then spend the next fourteen years of their lives working their assess off about it, but she does find a remarkably open-minded mundane linguistics professor who speaks fourteen languages and has a gentle personality that she will have no problems living with named Harry. They marry in a quiet civil ceremony, and when Lavinia is satisfied that she's built up enough mana and information resources, she stops taking birth control. 

 

She gets pregnant with twins. Blast it. Harry tentatively suggests reductive abortion, which she vetoes; the whole point of the endeavor is that she cannot bear to see her children die, and by the time they find out the fetuses have brains and can dream. No. She'll just have to work that much harder. 

Being pregnant is like being chronically ill, which is convenient, because everything being that much harder makes it that much easier to build mana. Labor may be the most mana-building-intensive activity she's experienced since she was a desperate senior.