You seem to be operating on a fair bit of misinformation, and should do some more reading. Cross checks are usually required, as there are a lot of really awful tutorials, like Instructables. The one I linked in reply #9 has lots of useful and technically correct ideas.
The DS18B20 draws about 1 mA in active mode and people very often power it by a port pin. A standard AVR Arduino port pin can safely supply 20 mA, 30 mA in a pinch.
You DO NOT need a power switch for the sensors or the radio.
Get each part of the project working independently before putting the bits together.