Hi! I teach 5th grade CCD with my Mom, and when the Holy Trinity came up, I did some research to find good ways to explain the three-in-one concept.
One of the best examples I came up with was an apple. All one apple, but made up of three parts, the skin, the flesh, and the seeds. All three are no more important than any other part, and they are all essential to an apple being an apple.
You can explain to them that the seeds are like God the Holy Spirit because He implants in us the seed of love of God at Baptism. He nurtures us and helps us grow from the very beginning. The fleshy part of the apple is like God the Son Jesus. He became man (flesh), and dwelt among us to bring us new life. The skin is like God the Father. He protects us from all the evils of the world, and keeps us from getting hurt. Then, you can cut the apple down the middle, and turn it sideways for them to see that the three elements make up the one apple.
Obviously, you can twist the reasons a bit to work with your class. I know this approach might seem relatively elementary, but if you such something like an apple to teach, they’ll be more prone to remember the lesson! I hope it works out for you!