Do you mean you want it to look like a thermometer? Or just that you want it to only display temperature?
Im not aware of a thermometer icon that you can use, so you would need to create one (should be possible if you use lots of different icons for each temperature, but its far from a clean solution!). Alternatively if you want to see some code where child devices are created and child devices are sent temperature information from the parent then take a look at my Heatmiser Thermostat device handler here.