Why were the Mothers of the Plaza de Mayo upset with Argentina's military government?

Their children were missing.

Their children had no schools.

Their children were hungry.

Their children had no homes.

The Mothers of the Plaza de Mayo (sometimes also referred to as the Grandmothers of the Plaza de Mayo) is an organisation of mothers of the disappeared people during the conflict in Argentina - they demand the truth about their children and in many cases they demand to be returned the children of their children - often the disappeared women were pregnant and their children stolen.

So the correct answer is : Their children were missing.