How would you explain an annulment to a child, particularly in the case of a couple that received one after 20+ years of marriage and three children?
It depends on how old the child is. If this is a young child, perhaps under twelve, and is not personally affected by the annulment, there is no need to explain anything to the child until he is older and perhaps is discerning whether he is called to marriage himself. A young child personally affected by the annulment might be told that his parents have completed a procedure offered by the Church to determine the status of the marriage in the eyes of the Church. Since the child has already seen his parents’ marriage adjudicated by the state, such an explanation probably will not alarm him. If this is a teenager affected by his parents’ annulment, perhaps one might give basic information about what an annulment is and explain that an annulment does not affect his status in any way; it only determines that his parents were incapable of entering the sacrament of matrimony at the time they were married. An adult child can be given similar information.
Annulments and the Catholic Church** by Edward Peters