I have only seen it once but I don't think it works like that anymore. Last night I was playing a family that had twin toddlers, and they were taken by the social worker. I wanted them back as they were spawn of the fireman and I wanted to test the pyromaniac trait. I switched houses and had a neighbor try to adopt, she got a new toddler girl.  I was looking through the hood and saw one of them through the neighbors window, I found the other one at another neighbors house. I cheated to get them back, all their relationships are the same, but I seemed to have borked them because they can't be aged up with the cake, only with testingcheats.