Works great. It uses the iCloud backups so you do need to make sure that is working. Also you will need to know the AppleID username and password. We track them in a password manager and ask our kids to tell us if they change their password.
If you use the Managed capabilities yes. We have a discussion with our kids that we use tools to help us manage the phone and track their usage. It helps to have a dialogue about it. Then they aren't surprised.
Not always and since the upgrade I can only read my sons response a can't see any incoming texts really sucks I contacted teensafe and they apparently can't fix it
All you need is their apple id and password if it is an iphone. It doesnt show previously deleted messages, it will show deleted messages from the day you get teensafe but not from before. If its an android then you will need your childs phone to install it.