It looks like you're using Internet Explorer 11 or older. This website works best with modern browsers such as the latest versions of Chrome, Firefox, Safari, and Edge. If you continue with this browser, you may see unexpected results.
Several different schema for developmental stages have been used by different psychologists. Step One for this assignment is to choose one, which your textbook may help you do.
The next step will be specifying the elements of your stage for which you will perform your design. This is where good keywords come from!
For example, Piaget's cognitive development stages include a "concrete operational" stage. There will be articles that discuss this stage directly, but you're much more likely to find results that talk about specific *elements* of the stage. These also make for easier design work. Look for ideas such as: