I'm happy to report that the work of both of these authors seems to be New Age-free. The kind of meditations Sr. Reehorst offers in her book are lovely and should be quite engaging for children. Sr. Jane Reehorst, BVM, author of the book Guided Meditations for Children was a much loved religious who died at the age of 88 on Nov. 10, 2013, at Marian Hall in Dubuque, Iowa. This file contains touching memoirs of those who knew her, including her brother and fellow sisters.
You can read an excerpt here.
Sydney Ann Merritt's books appear to be the same and are geared toward encouraging prayer in children, teen and adult catechumens. Merritt has had a long career in Catholic religious education and also serves as a court-appointed child advocate. She also contributed to the religious-education curriculum Celebrating the Lectionary.
What distinguishes the guided meditation of Sr. Reehorst and Ms. Merritt is that it is directed toward Jesus and furthering our relationship with Him. This is drastically different from New Age guided imagery which uses mental concentration to achieve particular goals such as improved health, a better financial situation, access to some kind of secret knowledge, etc. Rather than relying upon the natural imagination, it seeks to induce an altered state of consciousness and relies upon the powers of the mind to influence one's perception and personal reality. You can read more about it here.
This is not to say that all books on guided meditation for children are good. I came across disturbing titles such as Faerie Guided Meditations for Children and Indigo Dreaming meditations for children so be careful with this!