The Holy Trinity, for children, can be explained as one God who exists in three "persons": the Father, the Son (Jesus), and the Holy Spirit. They are all equally God, and yet they are distinct from each other.
In our whole school Gospel reflection, we explored this mystery using triangles and also shamrocks. It's a concept that's also often explained using analogies like water (one substance in three forms: ice, liquid, and vapour) or an egg (one egg with shell, yolk, and white, each representing a part). However, it's important to note that no analogy perfectly captures the mystery of the Trinity.