Supplies
Your kids can make a board game with simple supplies found around the home and a few bits and pieces from your local craft supply store. For longevity, use thick, durable poster board for the game board. Provide markers, crayons, paint and colored pencils for decorating the game board. Write on index cards for easy game play cards. Consider laminating these to increase durability.
Use spare die or a spinner from an old board game to regulate movement, or have your children come up with their own method (consider flipping coins). Let your kids make their own game pieces cut out from small pieces of poster board or made from modeling clay. Also consider using small toys, buttons, paperclips or coins as game pieces.
Provide construction paper, pipe cleaners, colored cotton balls and any other craft supplies you have handy for your kids to make three-dimensional "set pieces" (like a building or tree) to bring life to the game board.
Redecorate an old board game to match the theme of a favorite television show or movie as an alternative craft.
Design
Some games involve simply getting from point A to point B by rolling dice or flicking a spinner. Strategy games like chess, checkers or Risk allow more freedom in movement and focus on eliminating an opponent rather than getting to a certain point. Still other board games focus on collecting and accumulating with some sort of points-based system (like many trivia games or Monopoly).
Lay out these guidelines for your kids to help them decide what sort of games they'd like to make. For younger kids, a simple game of getting from point A to point B may be the easiest. Let them express creativity by making game cards that aid and impede players' progress.
Encourage your kids to add an educational element. For instance, they could make cards with math problems that have to be answered before the player can move forward. Trivia and strategy board games inherently contain educational elements on their own.
Rules
If your kids are creating a variation on a classic board game, use the original game's rules as a framework. For unique games, let your kids develop and write their own rules. Make sure the rules are fair and give all players an equal chance of winning. Strategy games require special care to achieve balanced game play.
Decide how important the element of chance will be to the game. Some games rely solely on chance (such as games where rolling the die or picking a card is the only way to affect outcome), some solely on strategy (such as checkers or chess) and some on both (such as Risk or Monopoly). Games relying solely on chance may not seem fair to older kids, as they can see they have no real means of controlling whether they win or lose.