Sometimes it's difficult to let children work with art materials with no goal other than their own personal response to the faith stories they know and their spiritual questions. It helps a lot to become familiar with art materials, choose and set up the supplies so they're easily available, and show the children how to use the materials (but not what they "should" make).
On Godly play
Godly Play: How to Lead Godly Play Lessons, by Jerome Berryman.
First of the newest series of books presenting the Godly Play approach to Christian formation, with plenty of real-life advice. Reflects another decade of Berryman's work with children and teachers since The Sunday Morning Handbook.
Godly Play: An Imaginative Approach to Religious Education, by Jerome Berryman.
The 'why's -- giving background and inspiration -- of this Montessori-based method of faith formation.
Teaching Godly Play: The Sunday Morning Handbook, by Jerome Berryman.
Wonderfully told stories, setting up the space, creating a sacred environment for the kids, and art response.
Dr. Sonja Stewart's Reformed faith keeps her focus closer to Bible stories than Berryman's more sacramental approach, and she has a greater emphasis on teaching rather than open-ended wondering.
Young Children and Worship, by Sonja Stewart and Jerome Berryman. With this book Dr. Stewart first put into publication many of the Rev. Dr. Berryman's Godly play stories, a seasonal structure, patterns for story manipulatives, and more. This, the "orange book," is so heavily used by some Godly play teachers that I've seen it rebound in a comb binding so it will lay flat and last longer.
Berryman developed his Godly Play approach after training under Sofia Cavaletti, who developed the Catechesis of the Good Shepherd. CGS is yet more Christ-centered and sacrament-focused, and less story-oriented, than Godly Play. CGS takes place in an atrium, requires more significant training of the catechists, and is more similar to classical Montessori education than Godly Play often is, in practice. Perhaps this is the other end of the spectrum from Young Children & Worship.
The primary books:
by Tina Lillig
by Sofia Cavalletti et al.
by Sofia Cavalletti
by Sofia Cavalletti; Rebekah Rojcewicz, translator.
Offering the Gospel to Children, by Gretchen Wolff Pritchard. Makes a great case for offering children the fullness of the religious experience without analyzing and predigesting it on their behalf, and tells stories of wonderful ways to do this, from All Saints to Palm Saturday to Corpus Christi. Great comments and recommendations about children's books and Bibles in the final chapters.
God, Kids, & Us: The Growing Edge of Ministry With Children and the People Who Care for Them, by Janet Marshall Eibner & Susan Graham Walker
Treehaus offers their hard-to-find books on children's spirituality
The Center for Children and Theology, which offers Catechesis of the Good Shepherd training, also supports research, offers retreats, and publishes some great material, some of which is available online.