How do I prevent someone from switching off mobile data from control center in iOS 11 while the iPhone is locked?
Do not worry, it is still possible.The way is a little long. Just follow these steps:


Go to Settings > General > Restrictions
If you have setup Restrictions, enter your Restrictions passcode. If you have not set it up yet, enter a password (twice) and enable Restrictions. Make sure you will never forget this passcode; it is required even when resetting the phone. If you forget it, not only will you be disallowed from entering the Restrictions section, you will also not be able to reset your iPhone.
On Restrictions page, go down to the “Allow Changes” section and click on “Mobile Data”.
Set “Mobile Data” to “Don’t Allow Changes”. Leave the “Data Plan” to “Allow Changes”. 
Open control center once and close it.


Now, the control center widget will not toggle mobile data and you can still go to settings and disable it while the iPhone is unlocked; just like you were doing till iOS 10.3