Yennefer’s want for a child is because she desperately wants to be loved unconditionally & believes the only person who could do that would be a child off her own.
Throughout the story she learns that is not true, she finds people who love her and she loves them.

Her greatest fear is that she is unlovable, she spends decades isolating herself so she doesn’t have to face rejection.
Meeting Geralt and Ciri opens up a part of her she has locked away for most of her life. It proves to her that she is worthy of love and she has the ability to

love others completely without fear of rejection or judgment.
Her story’s not about having a child, her arc doesn’t revolve around finding a love interest
Yennefer of Vengerberg’s story has always been about her recovering for her past & learning she is worthy of love.