Start with the people who will tell the story.
Write down who wants a substantial role, who would welcome a few lines, who prefers reading and who would enjoy a group song or dance. Include your available adult helpers. That picture is more useful than choosing a script by a single cast-size number.
Choose the kind of story your group will enjoy.
For an imagined sky adventure, explore One Small Star. One Small Star connects its characters' discoveries to Christ. Explore the complete collection for other stories and settings.
Check the practical demands before assigning parts.
Read the staging notes, mark scene changes and list props. A shadow-panel effect, a costume reveal and a reusable doorway require different preparation. Decide how songs will be led and check permissions for your chosen arrangements and recordings.
Read it aloud and time your choices.
Page count is not a running time. Read the script aloud, add the actual lengths of your songs and dances, and allow for entrances and transitions. Use that rehearsal estimate when planning a service.
Make a casting version for your group.
Share supporting speeches where useful, then update names, pronouns and entrances in your rehearsal copy once your performance licence permits it. Keep a master list of changes so everyone rehearses the same version.