My concern would be, that it would have to be something that only works inside your establishment. Otherwise, you'd have dozens of kids sit outside the restaurant, download the app on their phones and order up hundreds of $ in food, then drive off to the next place.
That being said, I agree with the person who said that an iPad is probably best, because people "eat with their eye's" and want pictures. Cost prohibitive though, if you have a 30+ person place. Plus the maint on them, wiping/sanitizing/charging them after each use, needing 3,4,5+ per table.
I could see a "tablet" type that stays at the table, that goes into a docking station after use. Again, they would have to be pretty sturdy, drinks spilled on them, kids drop them, get used as a coaster.
Maybe a touchscreen monitor at the table? I think its a great idea, it will take some time though to integrate it into our culture.