I am finding more to study and discuss than to actually do in celebration of this feast. The facts surrounding St. John's death are difficult to explain to young children... though they can be carefully summarized. There is much in the life of St. John, however, to explore as a background to his holy death: his role as precursor of the Son of God, the fact that he was the nephew of Mary, the son of St. Elizabeth, the cousin of Jesus, his life of simplicity and sacrifice, all climaxing with the ceremony of Christ's baptism at his cousin's hands.
Here are some resources for the day:
Information about St. John the Baptist, in generalBiblical reference to John's condemnation of Herod and Herodias' unlawful marriage: Luke 3:19, Matthew 14: 3-5.
Biblical reference to Salome's demanding of St. John's head: Matthew 14: 6-8
The conception and birth of St. John the Baptist are recounted from the Douay-Rheims here.
Here is a list of all the places and things St. John is called upon as patron.
Here are amazing photos of the head and hand of St. John the Baptist, first class relics both apparently held by the Muslims in Istanbul, Turkey.
On the plaque by the relics found at the Treasury of the Topkapi Palace in Istanbul - Turkey - can be read:
"The skull of St. John the Baptist was originally in the possession of the Byzantine and fell into Ottoman hands after the conquest. This relic was presented by Mehmed II (1451-81) to Mara Despena, daughter of the Serbean King.
Subsequently it became the property of Cezayioli Hasan Pasa and after his death brought to the palace around 1790."
"The (right) arm of St. John the Baptist and its case belonged to the Byzantine prior to the conquest. In 1484, Bayezed II (1481-1512) sent it as a gift to the knights of Rhodes. It was later discovered in Lefkose Castle in Cyprus and brought back to Istanbul in 1585"
A piece of St. John's skull is also reportedly venerated in the Cathedral of Notre Dame in Amien, France.
A good discussion on the understanding of relics in the Church can be found here (New Advent)
and here (exerpts from Canon Law on the subject).