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.