Teaching about the Trinity may be easier in Sunday school than in seminary. Because the Christian belief that there is one God in three persons is a deep mystery. Receiving a mystery is often easier for the children than for the highly educated.
What the Trinity is not
Depending on where in the world you are, it may be necessary to explain what the Trinity is not. Many Muslims believe that the Trinity is God the Father, the virgin Mary, and Jesus the son of God. This would mean that Christians make Mary a God, and this would be idolatry. This is an ancient misunderstanding. Mary was a special woman, chosen in spite of her humble status, who was given the great honor to give birth to Jesus. But she was not God.
The Trinity also does not mean that Christians have three Gods. Christianity is a monotheistic religion, believing only in one God who is ruler over all.
A secret
I think first of all it is important to explain to children that the Trinity is like a secret. We can see part of it, but we cannot fully understand with our human mind the mystery of God.
However, there are different ways you can try to explain. For example, God the Father as creator was over us. But in Jesus, God became one of us, and live with us. When He returned to God the Father, He sent the Holy Spirit to be ‘God in us’.
You can also explain how God is holy and we cannot be His friends because we have done so many things wrong. It is the Holy Spirit who works in our hearts to bring us to repent or say sorry. And it is God the Son who paid the price for our sins by being the Lamb that was sacrificed. This is how the Trinity works to bring people back in communion with God.
Another way that some people find helpful is to compare God to water. Water can be solid (ice) fluid or steam. But although it looks different, it is still water. In a similar way, God can be one person and yet three. Again, this is only a partial way to explain, but it may help some students.
Study the Bible
If the children are older, it can be good to study the Bible with them and show key verses on which the doctrine of the Trinity is based. A helpful list you can find for example here.
The Trinity is not a mathematical formula. But it is the picture of God that the scriptures paint. Without God the Son, there was no salvation. Without God the Holy Spirit, we would unable to recognize the truth.
Lastly, it can be a good practice to teach the children to memorize 2 Corinthians 13:14 and say it together after each meeting: “May the grace of our Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit be with us all.”