The UUA WorshipWeb has guidelines for writing your own prayers, meditations, and blessings. See the activity Writing Family Prayers in Families, a UUA Tapestry of Faith curriculum for youth; participants use Rev. Gary Smith's framework of five elements a prayer can contain.
Two Skinner House books with worship resources, available from inSpirit: The UU Book and Gift Shop, are A Child's Book of Blessings and Prayers by Eliza Blanchard and Rejoice Together: Prayers, Meditations, and Other Readings for Family, Individual, and Small-Group Worship edited by Helen Picket, which provided some of the prayers and meditations in this session.
Rejoice Together Prayers, Meditations and Other Readings for Family, Individual and Small-Group Worship, Second Edition
By Helen Pickett