Pope John Paul 11 used his power of Ex Cathedra so as no one could speak against it within the Church. This fact is not spoken about, especially by those that think they are pope for a day. The amount of resistance and opposition can sometimes be a good indicator of something spiritually good. Good on you and your family. Love displayed in loyalty goes down big in Heaven!
Actual Ex Cathedra statements are extremely rare, and there have not been many if I recall correctly. They are used to definte a doctrine which has been called into question. The last one I can find is the Assumption of Our Lady in 1950...I don't think JP II ever used it, and I certainly may be wrong, but could someone find the statement? Divine Mercy as a devotion is not the sort of thing that one would make an Ex Cathedra statement about. He encouraged the faithful in very strong language to adhere to this devotion, and named Divine Mercy Sunday as the "Sunday within the octave" or in the old rite "LOw Sunday."