It’s also cool if he or she doesn’t, but the reason why she’s saying no should be a character driven one (preferably not with too angsty a motivation). Right now, especially in YA, it’s become a common character trait for a female action hero to not want children. It imposes this belief that girls in action roles will wither up immediately if they’re exposed to any traits that are remotely feminine. It’s a repetition of the idea that strength – be it intellectual, emotional, or physical – comes from being as close to male as possible. A girl can kick ass and still want to be a mama bear one day, just like a woman can hold off on children because it’s “not the right time” as opposed to “ever”.
A man can want to raise a brood, be a stay at home dad, and enjoy baking in the kitchen. The same man may also be a police officer. A little girl may love Barbie dolls and Breyer horses, the same girl may also be raised in survivalist cult and capable of gutting a man six different ways with a knife. The dichotomy between the two doesn’t need to be a commentary or symbolize her loss of innocence, her inner femininity, or her “softer” side. It doesn’t have to mean anything at all. Liking handbags doesn’t have to be a sign that your supernatural protagonist just wants a normal life, just like being into fashion doesn’t automatically make one shallow.
Try to stretch outside stereotypes for characters.