The solution: I've only found one so far that fixes the issue after it hits and it is Overwatch and ErrorTrap. I can still do elimination tests but it takes up to a sim week of waiting. So if anyone's experienced this bug and has identified problem pieces, lemme hear it. Nothing shows up as corrupt or even a duplicate/conflict on Delphy's dashboard and yet the problem persists. I have been working to figure out the problem packages but it seems to be multiple ones and even those that I've used without issue in the past ( Colorbox's elder female clothing) or content that's reasonably popular now and not reported as problem causing (Ifcasims hair). The EPs I have on or off don't seem to matter. It happens in multiple saves (all my testing has been in a fresh Moonlight Falls because it tends to hit the same few pre-made households within a week, but I also ran into it in Bridgeport, Sunlit Tides, and Appaloosa Plains in the past). And that I don't have a good cause or solution for it.

Even worse is that the households they are in cannot be made active, and if attempting to move anyone else from the household into them, will cause the game to be stuck at 'processing' the move. If I install MasterController and go to Status -> Personal for that sim, they are listed as 'out of world'. But they won't show up if invited over for whatever reason and are essentially not in the world at all.

They are still technically a part of their original households. These sims are still in relationship panels (albeit with a blank portrait), show up on party invites if stuck as children, can be chatted with on the phone, etc. So to expand on the title: this isn't a case of those kids getting culled at all.

Sims 3 cc hair toddler