I was playing with a friend and she mentioned that she had walked away from her game and it was at the Manage Worlds area upon returning when she had been in-game. She checked the log and saw where the mom died and the child was taken. According to my friend the parents weren't together anymore and she hadn't tried to maintain a friendly relationship between the 2, though she had the other parent begin one with the child as a toddler.
I feel like if the single parent dies then if the other parent has an established relationship with the child (preferably more than acquaintance) then the parent should be given the option to take on the child. If the other parent doesn't then anyone else that may be related to the child and has a relationship should get the option.
I have a sim that is married and her mother just became a vampire so if she dies and her wife dies also her mom should be given the chance to raise the kid.