The Sims does everything it can to try and "simulate" real life. In this case, a recent glitch has resulted in spawning what looks like a social services worker. This social services worker proceeds to take the kids away if you're not doing a good job at looking after them in your game. The Sims Wiki says that the social worker "will take away a baby, toddler, child, or pet if their needs drop too low, or in later games, if there are no more older Sims remaining in the household (i.e. due to death). They also will come to deliver adopted kids to their new family."
Numerous players are encountering this new glitch when their toddlers go to pick up a tablet, and then won't put it down, which results in them eventually disappearing into nothingness.