Paul,
Does this happen when the screen turns off? It might be a power-saving function is cutting off the WiFi signal to save power when the device sleeps. Try updating the devices while they're plugged in to see if it will keep the connection alive. If that doesn't work, you could try resetting the network settings on each iOS device. Settings > General > Reset > Reset Network Settings, just be aware that it will lose all the WiFi names and passwords it's memorized and set the network settings back to default, so you'll have to reconnect to WiFi, Bluetooth, etc..
Hope this helps!
-Shaun