You might want to have your children write a report about their findings. Choose a theme for your games. Look at the games in your own cupboard and see what the themes are. Monopoly is about buying and selling property. Scrabble has a vocabulary theme.
Some games are about shopping, or history, or playing.
If you are making an educational game, choose the subject and narrow the focus. In other words, a game about American History might be too broad. A game about the Civil War is manageable. Decide the age range of your players. Look at other games and see how that will affect the way the game is planned.