Luke Wroblewski’s critique of touch-screen GUIs for apps for toddlers reminds us that children do not react to interfaces as adults would. Most adults (well, not me) have at least some patience with computer interfaces. Modal dialogs are tolerable. Buttons are generally understood to require only one action (ie. click or tap) before they respond.
Toddlers don’t come built-in with hang-ups about computer interfaces. They don’t know about progress bars, single taps or modal dialogs. Apps designed for toddlers should take this into account. Avoid clunky widgets like modals. Tappable areas which launch an action should ignore taps until the action is complete. Otherwise, enthusiastic children will tap away and wreak havoc.
These thoughts fall in line with an idea I had a few years back of a “comfy” user interface. Comfy interfaces are predictable and a pleasure to work with. Perhaps having children test your app may catch some bad designs and lead to more a comfortable user experience.