Sadly this is around for a very long time. Best solution would be, if the game wouldn’t allow to move a mother to her new husband. The name could be changed by using the ultimate make over for 2 LPs, but you don’t have many, yet.
Since you can’t remove the mother, the little one can’t be adopted by another family either. This would only work with an orphan.
For now you could whistle over the toddler to her mother’s new home, so she’d not be all alone. On level 17 you’ll complete the quest “Preparing for preteens” that enables you to age her up to a preteen, and on level 21 there’s a quest to age up preteens to teenagers. After completing that one, you can age her up to teenager and then move her in with her mother. Or the other way around, if the mother isn’t married again.
On level 22 you’ll need a teenager for another progression quest. So it would be good to stock up enough LPs for the birthday cakes. E.g. you could have most of your Sims cooking in order to gain LPs, the toddler can possibly help by doing the playhouse hobby.
This would be my plan to continue without removing the toddler. Maybe, you could go continue this way, too?