Have your children analyze the games. Look at the recommended ages. 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.