Video baby monitor.
We've used it camping a few times. We have. Bluetooth one. You just have to stay within a certain range.

We use a Summer Infant In View Digital Video Monitor.  It is not Bluetooth, it has a camera that sends a signal to a monitor.  If it was Bluetooth we would be able to view it on our phones.  There are models like that, just at a higher price.  Our model claims to have a 600' range.  Being outside, the wind and trees can affect that range.  I have found that we can usually go 2 maybe 3 sites over and still get a signal.