I played Pokemon (both the card game and video game) as a kid and Magic: The Gathering as an adult. I advise extreme caution; both of the card products are basically scams. Artificial rarity can lead to spending truly insane amounts of money.

I would look for non-trading non-collectible alternatives. One card game I enjoyed as a young adult was Dominion, which has a bunch of expansions and therefore can get pricey, but it's really nothing compared to MtG or Pokemon where new cards are constantly released and (most of) the old cards lose value. For greater competition in the strategy space I would recommend classic board games like backgammon (which IMO is the most fun), chess, checkers, etc.

Assuming that your child has his heart set on Pokemon TCG, I would look into proxying, ie printing up the card images yourself for (almost) free.