Cheap way to communicate between microcontrollers with rf
The cheapest way of rf is nRF24L01+ by Nordic semiconductors.
These are 2.4 GHz, range can be high (even without external antenna), as long as you keep the bandwidth/baudrate low. Note to buy the + version, since these are updated/newer. There are also versions where you can add an external antenna, but probably you don't need that.

These have a feature to put 6 in a master/slave mode, you probably need the opposite (1 micro sending to 6 others), but that can be easily been done, by e.g. every second (or more) asking for a response. Maybe something smarter in case you want to use batteries.