Children usually think of God the Father first whenever God is mentioned. So when talking about Jesus, let them know that Jesus has always been God and He became a man without stopping to be God. He is 100% God and 100% man at the same time. Colossians 1:13-18 and Philippians 2:5-11 can help here.
Be careful of referring to the Holy Spirit as "it." Talking about Him as the "helper" that Jesus sent can help children understand that He is a person, not a thing. He is the One who lives in each believer from the time of salvation. John 16:7-14 and Titus 3:4-7 are good passages on the Holy Spirit.