If they were young children, I think we would have heard about it. As we did with William, due to extensive searches etc.
If they were teens, who knows. Teens, I believe, are sometimes classed as runaways, when they perhaps may not be. Whether they are in a foster situation or not.
Though I am not sure how revealing that a child is a foster child is going to make any difference to the situations. It has made no difference to William's situation, and never was going to. A fruitless exercise imo ... other than Allanna Pearl Smith - and everyone else - can now speak about it publicly.
There are good foster parents, like Williams. There are good biological parents, like mine.

There are also bad parents in both categories.