Thank you for this question.
Women are generally not permitted to immerse during the day except for very extenuating circumstances. The discussion begins with concern that the woman would immerse and have relations on the seventh clean day, prior to the end of the shivah neki'im. On subsequent days, a key reason for concern is lest others (especially, but not only, daughters) get a misimpression of the halacha.
Even so, a kallah is considered an exceptional case, because in most circumstances, wedding timing will not allow for her to be secluded with her husband—and have the possibility of relations—prior to nightfall (learn more here). This leniency applies to a kallah with children as well. Many halachic authorities express a preference for any kallah to immerse at night, but allowance for a kallah's daytime immersion does not depend on her being childless.
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