Simple network protocol to communicate between N slave arduino and 1 central arduino?
You have quite a few options available to you as your requirements are filled by quite a few protocols, I am not going to list all. You can use things such as I2C, CAN, RS-485, RS-232. 

You can find a few good examples from this EE.SE post from a search.

The CAN option is quite a good one for your application and is easy to set up and there are quite a few libraries available for the common chip MCP2525 and MCP2551. Yo can also get a 'shield' with this functionality from sparkfun.

The other options are discussed in the linked EE.SE post with regards to RS-485.

I2C can be implemented with the Wire library of arduinos and other libraries.

You can use RS-232, but I don't think it will be immune to noise at the distance you want, but you can still try. What you can do is use a multiplexer, such as this one CD74HC4067, and use it to multiplex the TX line from each node to the RX of the master.

From SFE's site:


  For example, you could use it to connect the TX pins of 16 devices to one RX pin on your microcontroller. You can then select any one of those 16 devices to listen to. If you want two-way communications, you can add a second board to route your microcontroller’s TX line to 16 device’s RX lines. By using multiple boards, you can create similar arrangements for I2C, SPI, etc.


You will probably want to add some sort of checksum/CRC to the data you send over the network too.