Firmly I believe and truly
God is Three and God is One
So starts John Newman's hymn from 1865. I may believe it, but the Trinity, God in three persons, remains a mystery which is hard to comprehend and explain. Sharon Ely Pearson and Carolyn C Brown have ideas which you may find useful in exploring the mystery of Trinity with children, and Martyn Payne has plenty of activities for you to try. Links to their sites are on the right.
With young children, this song to the tune of Frere Jaques might be helpful:
God the Father, God the Father
God the Son, God the Son,
God the Holy Spirit, God the Holy Spirit,
Three in One, Three in One