It has been a big HOW for me to explain to kids the Christian doctrine of the Trinity that teaches the unity of Father, Son, and Holy Spirit as three persons in one God. As an adult Christian, I receptively accepted it, but a mind's child cannot grasp the eminence of the Holy Trinity. They have lots of questions that are hard to break down in their level.
During one of our discipleship group meeting, my leader explained the Trinity to us in a very simple and fascinating way. The Holy Trinity of Father, Son and Holy Spirit, just like how God created man as stated in Genesis 1:26,
"Then God said, "Let us make man in our image, in our likeness ... " (New International Version)
In the same way we humans have body, soul and spirit. Our body that enables us to do physically do the our work here on earth, our spirit that allows us to praise and worship Him and our soul that will remain after death. And though we have this 3 different part in us, we remain one person. Interesting!
If the first explanation is not convincing to you, I hope the second illustration makes the idea crystal clear ;)
She compared the Trinity to an egg which is composed of a shell, egg white and yolk... 3 different parts thats makes up one single egg. Similar to God having three persons: Father, Son and Holy Spirit... 3 different persons that makes up one single Higher Being.