Let's face it. Truly understanding the Trinity is difficult even for adults. That's why it's important to start introducing the concept to your kids at an early age. There are a number of books for helping kids understand the Trinity - Big Thoughts for Little Thinkers: The Trinity and 3 - in - 1 A Picture of God are two of the best, in my opinion. And there are plenty of thoughts about illustrations that help kids understand the concept. But really the best is one of the first that was ever used: The Shamrock.
Shamrocks fit in nicely with St. Patrick's Day and offer an opportunity not only to learn about the Trinity, but also about a great man who lead many people to God. One of the ways he did this was to show pagans an ordinary shamrock or clover. Like the shamrock, with it's three separate leaves, St. Patrick said, God is one, yet three. Simple and to the point.
One of the crafts my daughter can't wait to do each year is a simple shamrock labeled with the Persons of the Trinity. We use this template. All my daughter has to do is cut out the shamrock pieces (if your child is younger, you can do this ahead of time) then write in "Father," "Son," and "Holy Spirit." If your child isn't at all ready to write, this template might be a better choice, since it requires only pasting. Here's another simple variation - and a more complicated shamrock for older kids. Of course, you don't really need a template. You could just cut a shamrock shape from paper.
But whatever method you choose, let it lead to a conversation about the Trinity and who each Person in it is. It's a great way to start learning one of the more important - yet difficult - parts of the Bible.