I started having this problem a while back, and it's intermittent which doesn't help tracking down the source or the cure.

First of all weird children have been randomly turning up in families. I don't think they're being born, because even though I've set Death:aging false in NRAAS, they turn up as children, not babies. They all look alike, when they start appearing, including having the same hairstyle, and clothes which use the "nude" texture. (In other words they all have pink bow hairstiyles, with blonde hair, and they are wearing some jacket trousers combination mesh, but colored as if they were not wearing anything.) (Further update. I just did a "reset everything" . The mutant children are still there but now they are all wearing the cowboy outfit from Pets.) The family trees on these families show them as children of the parents in the family. I noticed that they originally turned up after I had a Sim use one of the Time Machines. This resulted in a child clone of the child appearing. She seems to be all right, looks acts and is dressed like a normal child.

At the same time another problem cropped up. When I try to change active families it will not let me do so. I chose the family in Edit Town, tell it to make the family active, and then the game loads the previously active family. These aren't always the families with the weird children--just now it won't let me load any family--but it started at the same time and I suspect the problem is related.

I suppose I should just write the neighborhood off as corrupted, but I'd like to salvage it if I can....

Try deleting .package files in you sims 3 folder. If your playing in a store (or possibly custom) world try deleting the world caches related to the town your save is in. Also check your saves folder and delete the .bad folder for your save if there is one.

I don't have any bad saves at the moment. I'll try deleting the world cache though and see if that helps.

Edited to add: I finally gave up and moved everyone to a new neighborhood, and so far the problem has not reoccured. I used nraas to kill the strange children. If it comes back I'll try deleting the world cache.