My answer would be:
"Look, son, many, many years ago, after the birth of Jesus of Nazareth, the Kings came from the East to pay homage to him and to give him gifts of great symbolic richness: gold, incense and myrrh." The kings of today do not exist, but all parents want you to have gifts like Jesus, so tonight, we play with the kings and if you have fulfilled all your obligations, we want you to make memories that can be an illusion.You now know the truth, you are older, but if you want we can continue playing the Three Kings, because we all like others to think about us and give us gifts, your sister who is smaller does not know anything, what we have talked about is our secret. "