The camcastviewer app lets you watch in real time the video output off any device that has the companion camcastlens app. It allows up to 4 simultaneous connections. So you can setup 4 devices to create a mini surveillance network on the go.
Once you download this app go ahead and download the camcastlens app on the devices you want to use as the camera.
With Family Sharing set up, up to six family members can use this app.