Every user should have their own AppleID. This is essential to maintain intended separation of activities and to avoid unintended sharing of sensitive and data. Done properly, you’ll not have any issues with content or information being inappropriately shared between devices.
Perhaps the cleanest and most effective method of resolving this issue is to erase the device and set-up again “as new”.
What to do before you sell, give away or trade in your iPhone, iPad or iPod touch – Apple Support
You should find this information to be helpful with setting-up an AppleID for a minor - and considering appropriate controls:
Apple for Kids - Official Apple Support
Help your child set up an iPhone, iPad, or iPod touch - Apple Support
Family Sharing and Apple ID for your child – Apple Support
Use parental controls on your child's iPhone, iPad and iPod touch – Apple Support
Otherwise, removal of an email account from one device will not impact email on other devices:
Settings > Mail > Accounts > [Mail Account] - Delete Account
You should note that this alone will not resolve other issues associated with sharing an AppleID.