So to expand on the title: this isn't a case of those kids getting culled at all. 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. They are still technically a part of their original households. But they won't show up if invited over for whatever reason and are essentially not in the world at all. If I install MasterController and go to Status -> Personal for that sim, they are listed as "out of world". 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. And that I don't have a good cause or solution for it.
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). The EPs I have on or off don't seem to matter. The cause: unknown, possibly CC. 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). Nothing shows up as corrupt or even a duplicate/conflict on Delphy's dashboard and yet the problem persists. So if anyone's experienced this bug and has identified problem pieces, lemme hear it. Or maybe this is a red herring. I can still do elimination tests but it takes up to a sim week of waiting.
The solution: I've only found one so far that fixes the issue after it hits and it is Overwatch and ErrorTrap. Well of course. But the thing is that I usually don't play with them for dynasty reasons and the solution isn't perfect anyways:
1. It seems to only kick in when I make the affected households active. I don't play mutiple households so this isn't just something I naturally would do anyways.
2. It doesn't always work (in my first save where this hit, I had both mods and others in the NRaas site and it wasn't able to fix my town's lost toddler, so I ended up transferring her genetics to a random "pod person" sim and manually attaching the new sim to her old parents and so on...it could have been a different issue but it had all the hallmarks of this)
3. If it does work, it has altered the face of the affected sim on more than one occasion. Not optimal.
4. It's better to not have the problem at all than to have two mods scramble to fix it.
For what it's worth I have repaired and fully reinstalled my game on multiple occasions in the time I've experienced this (for other reasons), I'm currently working from a new "The Sims 3" folder, and I've been on patch 1.69 since its release.