Might be easier and cheaper to just buy a bunch of cheap tablet computers and use standard wifi networking. Then you'll just have to write software for the devices and host. Unless you aim to make them pay via those? (security risk). Also they could just run off with the tablets, so perhaps securing them to the table somehow - risking spilling things on them, or smashing them. Then there's hygiene etc...
Building a system from scratch will be time consuming, expensive, and still won't overcome the pitfalls of the above idea.
As for just being curious, each device would be an embedded computer, often ATM's and in-shop touch screen displays actually run windows, and custom software, because its easier to buy them as conventional small form-factor PC's rather than custom builds. That also means all the standards for connections have been worked out (VGA for the screen, power supplies, networking etc..). These are hardly portable though, and require a power and network connection (you could go wifi).
I believe some trendy restaurants in London are doing this, but I suspect its more of a gimmick than to increase profits. Customers would still want waiters and waitresses around, because its all part of the restaurant 'experience', that and customers can't ask for specifics like 'no potatoes'. Sometimes, even when the technology allows, and seems like it should make life easier - it can't replace old methods