I'm about to place my first foster and don't relish having to notify the other potential adopters that i've chosen some other family evem though they would be great as well.
Any advice for a first timer?
tell them just that...that this particular family was the "perfect" fit but you will help them find "the one" for them.
We never let fosters pick adopters. They got to "weigh in", but not make the final decision.
I don't think an explanation is needed.