To provide more specific techniques, please re-post with the following information:
- The name of your VPN service provider.
- The model of your iPhone and your daughter's.
- The version of iOS running on your iPhone and your daughter's.
With this information, many can offer more targeted advice on how to disable the ability to turn off the VPN on your specific device and VPN service provider.

Here is some general guidance on how you can enhance the security of your VPN on your daughter's iPhone.
- Choose a VPN app from a trusted provider with a good reputation for privacy and security.
- Most VPN apps have an option to automatically connect to the VPN when your iPhone starts up or when you access certain networks. Enable this feature to ensure that your VPN is always active.
- Set a strong passcode or use biometric authentication (such as Touch ID or Face ID) to protect your iPhone. This helps prevent unauthorized access and reduces the likelihood of someone disabling the VPN.
- Some VPN providers offer a mobile device management (MDM) solution that allows you to create a profile with restricted settings. This can prevent users from disabling or modifying the VPN settings on their devices. Check if your VPN provider offers this feature.
It's important to note that disabling the ability to turn off the VPN on your iPhone may have unintended consequences or restrict your ability to troubleshoot network issues. Always consider the implications before implementing such restrictions.