A Christian believes in one Living God, one Deity, that shows Himself in three different ways. Trinity is never mentioned in Scripture and is arguably the most difficult theological concept to comprehend. Still, our kids hear it and we must do out best, with grace abounding, to describe this complex truth to young children. Depending on age grasping anything they cannot see will be especially hard. More-so, logical children will be frustrated when we tell them 1+1+1=1 when describing God. There are plenty of illustrations pastors have used over the years to communicate this truth. None are perfect, but we do our best.
The trinity is made up of three parts: 1. God the Father or Creator 2. God in Flesh - Jesus Christ 3. God in Spirit - Holy Spirit. Each aspect is a different expression of the same God. God the Father is Almighty, sovereign over all Creation, Lord in Heaven and on earth. He remains unseen, but we see His work. Jesus Christ is God in the flesh, Savior Messiah who walked the earth, and restores sinners back to Himself (God) through His death and resurrection on the cross. The Holy Spirit is the Spirit of God left for all of us when Jesus returned to heaven. (Acts 2) The Holy Spirit is left on earth to be our Advocate and dwell in us so we are evermore transformed into His likeness.
One illustration I find helpful:
I can talk to my mom on the phone and it's my mom. I can see her in person and hug her and it's my mom. I can say an exact phrase just like she would, hear her influence on me when making decisions even as an adult, or pull a face that resembles her and it's still my mom showing up through me. In the same way we can talk to God the Father without seeing Him (prayer) and see all that He has made (creation). The disciples could touch Jesus' cloak, scars, hair, and hug Him in person- God in flesh. Christians can quote Scripture, hear God's direction and voice as we go about our day, or mimic Christ's actions and it's God's Spirit coming out of them.