They told this story at the Primary open house last spring in the Counselor¹s Session.
Unfortunately only the President's message is published in it's entirety.
The counselor's session just has "notes". Which only tell where you can find the song (Don't Ever Forget to Pray  April 2006 Friend magazine). So you¹ll have to rely on my memory and notes:

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Cheryl Lant, the General Primary President, spoke about a recent meeting she and her Board had with the First Presidency.

As they parted, Sister Lant asked President Hinckley if he had a message they could deliver to the children of the church as they traveled around the world to different Primaries.
He said, “Yes. Tell them to never forget to pray!”

Sister Elizabeth Ricks, a member of the General Board, thought about this message as she drove home from that meeting. As she drove, a melody came to her mind. She stopped and jotted it down on a receipt, the only piece of paper she could find.

The results of that meeting and her inspiration are published in the April 2006 Friend.“Never Forget to Pray” is a short, catchy song containing the council President Hinckley has for our Primary children today