For my 6y.o. – the key ideas of trading card/deck games

My six year old son has been creating trading cards, hand drawn cards of magical beasts with powers, attacks and defenses. They've become a hit at school. He's set up a small stall and he's taking orders on a little index card from students and teachers. At the end of the day he comes home with the list "Morgan wants infinite deathmage..." and starts drawing.

But neither he, his friends nor I have every got into trading card games, any of the gamut from pokemon to magic the gathering. So the cards are great but the game is dull, basically just rock/paper/scissors.

I know some will say the meta game of r/p/s is infinitely deep, but what I really would like your help with is an intro to how trading card/deck games work. I can go online and read the rules of game X, but I'm turning to you, who have the perspective of deep investment of many years in multiple games, to ask what captivated you, and with that perspective what general architecture patterns have you come to see? what makes them fun for you, and what basic ideas he can think about introducing that make choices and strategy interesting without making the rules too complex for kids to understand?

Thanks

m.c.t