In Double Down, you play one card and then draw one card. When you play a card, you add the value on the card to the current total and then announce the new total. For example, the current total is 9. If I play my 2 card, then I announce that the new total is 11. But watch out, that move will hurt you! You do not want to lay down a card that causes the total to add up to a double number, meaning 11, 22, 33, etc. (You don't want to Double Down). You also don't want to cause the total to reach 99 or higher.
In the rules, doing either of those things costs one chip. If you lose all your chips, you are out of the game. Last person standing wins. I do not like using elimination style games in gameschooling. Instead, I put a pool of chips in the middle, and if someone doubles down or causes the total to be 99+, they take a chip. The person with the least amount of chips at the end of 3-5 rounds is the winner.
There are three types of special cards in the deck. The -10 lets you subtract 10 from the total. The x2 makes it so that the total stays the same and the next player has to play 2 cards. The square of arrows is a reverse. The total stays the same and play changes direction.
Double Down is great because the children are doing a lot of mental math, but are focusing on trying to stick the other players with a chip so that they don't end up with one themselves. The number cards are not all single digits. There are values from zero all the way up to 99 in this game, which is great math practice! If your children are not proficient enough to add double digit numbers in their head, give them some scratch paper and a pencil. No big deal. They'll get there with time and practice.
You can purchase Double Down from Miniature Market.