The code you wrote in the file is fine, just use it. You use code in a scene, just like I told you. Not in the file, you use it in the scene. I told you where. Just create the scene like I said, and place the code where I told you to, ok?
Here is what it does:
1) local CurrentTemp = luup.variable_get("urn:upnp-org:serviceId:TemperatureSensor1", "CurrentTemperature", 13)
This picks up the current temperature reading from the device with device ID 13 and stores it in a local variable
2) luup.variable_set("urn:upnp-org:serviceId:TemperatureSensor1", "CurrentTemperature", CurrentTemp, 10)
This takes the content of the local variable above, and updates it to the device with device ID 10.
In other words: It copies the temperature reading from the child temp sensor to the father thermostat device. Given they have ID 13 and 10 respectively of course.
This is the only thing your scenes does. Nothing else needed. No devices to be selected, no triggers, nothing, just code. The code runs when the scene does.
If you do the Startup Lua correctly, the scene will run whenever the temperature from device with ID 13 changes. Very easy.