Sum Swamp (paid link) is a great game for young children just learning to add and subtract. There’s just one problem: Because, as written, the outcome depends completely on luck, the game isn’t very fun. But there’s a simple way to modify the directions to add an element of strategy and make it a vastly better game.
The basic setup is that you move your game piece through the board based on the dice you roll combined with some space-specific operations. On each turn you roll two number dice plus an operations dice (addition or subtraction).
The strategy variant is simple: After a player rolls the three dice, the player may re-roll (once) any or all of the dice. Usually a player can improve his roll considerably by doing this, but occasionally a player may have to live with a worse outcome. This also speeds up the game. I’d limit the re-roll only to a primary roll. For example, if a player lands on a space marked “even” or “odd,” a player has to roll an even or odd number (respectively) to leave the space; I wouldn’t allow a re-roll in those special cases.
You could play the game with (up to) four young children with adult oversight. My wife and I play the game with our five-year-old, and we have him calculate the rolls for all of us. It’s really good practice for adding and subtracting numbers up to six, and, with the rule variant, it’s fun to play.