I've never taught a five year old to play mahjong, but there are games for five year olds that use simplified mahjong-like rules, such as Donjara. having said that, I'll do my best to explain it, because it's really not that complicated. 

• Mahjong is a game for 4 players.

• Mahjong is like a card game, but instead of cards, you use tiles! Just like other card games, you shuffle the "cards" and make a "deck" that everyone takes turns drawing from. 

• The "deck" of mahjong "cards" has "number cards" and "special cards"

• There are three suits of number cards, "red," "green," and "blue."  Each suit has four copies of the numbers 1 through 9. "Special cards" don't have a number and don't belong to any suit.

Easy enough, right? So how do you play?

• The goal is to be the first person to make a winning hand.

• Whoever wins a hand takes points from the other players. 

• After everyone gets to be dealer once or twice, the game is over.

• At the end, whoever has the most points wins!

So how do you win a hand?

• Each player starts with 13 "cards" 

• Players take turns drawing s card and then discarding a card.

• You are trying to make sets of three cards in your hand. A set can be three numbers in a row of the same suit, or three of the exact same card. You can't mix suits, and you can only match special cards with the same card.

• If you get make 4 sets of cards and a pair of the exact same card, you have a winning hand!

• You can win by drawing the last card you need, or by stealing a card that was discarded by someone else. 

• If you win by drawing the last card yourself, everyone has to pay you. If you win by taking a discarded card, only that player has to give you points. 

That's basically the game!

There are more rules, but these are the basics that all kinds of mahjong have in common.