Sunday School usually refers to a program in which children (typically younger than First Communion age) enroll in a religious education program that takes the place of Mass. Children are instructed in topics of faith, but do not celebrate the Eucharist, nor participate in Mass.
Children's Liturgy of the Word reproduces the liturgical, prayerful and ritual based environment of the Mass, but with developmentally appropriate readings and a homily experience geared for children. CLW includes the same prayers and readings that the larger Mass assembly is using.