There's no problem with your game. What's happening is that there is sometimes a delay between when the mom returns to work and the removal of the baby graphic from her arms. Meanwhile, there's a new 2-year-old running around the village. This is totally harmless, and the phantom baby will "disappear" when the mom decides to do something that requires the use of her hands (such as farming, fishing, laundry, etc.) or if you grab her and she "drops" the baby graphic.
