You could also just have it decide exactly when it wants you to find Waldo. Imagine you had a Where's Waldo ipad app that had 20 different "pages" and you could set the amount of time it takes to get through them all so you could keep your kid busy for a guaranteed minimum time. :)

Or even better, it's listening for you to say "ok, it's time to go in two minutes. Finish up," and it makes sure Waldo appears where the kid is looking just in time.

The downside is when your kid finds out he's going to murder you.