On the one hand, not telling them could lead to family feuds and hurt feelings when and if they later find out. On the other hand, telling them could lead to hurt feelings and feuds now. This is a situation where there is not necessarily a right answer that is generally applicable.
What to do depends on the individuals involved.
The article does make one very important point, however. Anyone who is chosen to be a guardian of minor children should be told beforehand.
Before making an estate plan that nominates someone for guardianship of minor children, it is important to make sure that the potential guardian is willing to do the job. An experience estate planning attorney can assist you every step of the way.