It used to be commonplace for children to learn Gregorian Chant, and there was a two-volume book called Plainsong for Schools.
I have scanned my old copy of the two volumes and cleaned up a few pages. Most of the work has now been done and they have come up quite clean after a little bit more work. The volumes could easily be printed by any publisher who wanted to reissue the book, subject to copyright clearance from whoever owns it, if anyone, probably Desclee of Tournai who seem to have licensed it to Rushworth and Draper who are no longer in business.
Not only would the books be good for children - they would also make a good parish hymn book.
If anyone is interested, please pass this information on or contact me.