Here is a good resource for teaching The Lord’s Supper to preschool children:
Emphasize that The Lord’s Supper is observed by baptized believers in Jesus. It would be quite okay to briefly mention what it means to be baptized (see Baptist Faith & Message, below).
Be sure to use the term “grape juice” or simply “juice” rather than “wine.” FYI, the Bible does not refer to it as juice or wine, but rather cup or fruit of the vine.
Note that the teaching plan recommends to not review the words of Jesus at the Last Supper while the children eat their snacks. See page 3 of the teaching plan for further explanation.
The Baptist Faith & Message has a brief summary of our faith statement on The Lord’s Supper and Baptism:
VII. Baptism and the Lord’s Supper
Christian baptism is the immersion of a believer in water in the name of the Father, the Son, and the Holy Spirit. It is an act of obedience symbolizing the believer’s faith in a crucified, buried, and risen Savior, the believer’s death to sin, the burial of the old life, and the resurrection to walk in newness of life in Christ Jesus. It is a testimony to his faith in the final resurrection of the dead. Being a church ordinance, it is prerequisite to the privileges of church membership and to the Lord’s Supper.
The Lord’s Supper is a symbolic act of obedience whereby members of the church, through partaking of the bread and the fruit of the vine, memorialize the death of the Redeemer and anticipate His second coming.
Matthew 3:13-17; 26:26-30; 28:19-20; Mark 1:9-11; 14:22-26; Luke 3:21-22; 22:19-20; John 3:23; Acts 2:41-42; 8:35-39; 16:30-33; 20:7; Romans 6:3-5; 1 Corinthians 10:16,21; 11:23-29; Colossians 2:12.