I wouldn't do corn personally - too tall and I am sure it will also walk away. I would plant something you need to "dig" up to get to i.e. potatoes, carrots (which are also decorative). beets, etc.. Maybe throw in some lettuce varieties & spinach that are pretty too - you need a knife to harvest those so I don't think they will walk away as easily as I can't see a kid picking a dirty spinach leaf and tossing it in their mouth. Beans could also be good - someone might pick a few but the time it would take to get them all would be prohibitive as they could get caught. Avoid strawberries & peas - too easy to pick & eat as you go.