Adults understand in the realm of reason, and reason can't really explain G-d. See What is G-d?.

Children understand in the realm of relevance, and that's what really matters when it comes to understanding G-d. See What does it mean to believe in G-d?.

One should therefore teach a child the following:

Just as everything you see, have or want was made by someone, so too this world and everything in it was made by a Creator: G-d.

the fact that G-d ought to be relevant in his life, regardless of his theological expertise, will remain with him forever

He even has a purpose for you.

G-d created you because He loves you, because He has faith in you. He has faith that you can take care of His world, and make it a better place.

Because G-d created you, He knows everything you think, say or do. And He is always there to help you. He shared the secret for perfecting His world in the Torah, and He wants you to master it, and follow in its ways.

Ultimately the child will grow up, and when he does, he will explore G-d with the intellect as well. And since you can't make a second first impression, the fact that G-d ought to be relevant in his life, regardless of his theological expertise, will remain with him forever.