In all seriousness, I think the best way to explain things like this to kids, is to come right out with it, but explain it in terms of, "Some people believe...." instead of, "This is how the Trinity works..."
When you explain matter-of-factly how something works, a kid will bend what they know to this new "factual" information they are being told. But you're not explaining fact, you're explaining faith. One particular interpretation of one particular faith. So, it's a little disingenuous to say what the Trinity *is*, when clearly, you don't know and are just grasping at anything that comes in threes.
Give them some credit, and explain it as a belief instead of a certainty. Now, they're thinking instead of memorizing. Maybe they will decide they believe it, too. If you do, it will certainly influence them. But let them come to a belief, rather than tell them what to believe.