There isn't currently a way to set an iPhone (or iPad) to require a passcode to allow powering off, without jail-breaking your device. It seems to be quite a popular feature request that Apple haven't implemented. For example, this 9to5Mac post from earlier this year. https://9to5mac.com/2016/01/28/feature-request-theft-protection/
Unfortunately if your device is turned off it will not be able to update its location. However as soon as the device is turned back on (assuming it is turned back on) it will resume updating its location. Thankfully, without a lot of effort, Find my iPhone can only be disabled by entering the appropriate Apple ID password.