Toddler is missing from household
@Ravenblackbird1 This kind of thing happens without mods, unfortunately. The game culls a sim, often a toddler, but some of the data remains for a while. That's why you see the toddler in the relationship panel but can't find her anywhere in town.
If you'd like to fix this, the only way, aside from reverting to an old-enough save that you don't have, is to use a mod. I would actually add several: NRaas ErrorTrap, Overwatch, and MasterController. ET and OW do their cleanup in the background; MC doesn't do anything until you use it to issue a command. You can find more information about all of them, plus download links and install instructions, here:
With the mods added, click on the house itself and select NRaas > MasterController > Add Sim, then find the toddler on the list and add her. If there's enough data left in the save to rebuild her, she'll be added, and ET and OW will help clean up any bad data. This won't prevent the issue from occurring again, but hopefully this particular sim won't be targeted, so to speak, the next time around.
I would also recommend using these three mods in general, for the long-term health of your save. However, they're safe to remove at any time if you choose to do so.
