How to set a password so my kid can’t change setting on her iPhone

A child under the age of 13 must be in a Family Sharing account. If you have not done so already, set up a Family Sharing plan.

Set up Family Sharing - Apple Support

Now to change the Apple ID from an adult to a child account, you will have to update the birthdate associated with that Apple ID.

Update the birth date associated with your Apple ID - Apple Support

You will now be able to use Parental Controls to restrict what the child can do, including changing account settings. When setting up the Parental Controls, you will set a password to restrict the child's access. The option is called "Use ScreenTime Passcode".

Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

Make sure your child does not have the passcode or Apple ID password for your device. If they no either of these, you will want to change them.

Set a passcode on iPhone - Apple Support

Change your Apple ID password - Apple Support

With respect to the first question, see "Set a Screen Time passcode" on this Support page:

Use Screen Time on your iPhone, iPad, or iPod touch - Apple Support

The Related Article also mentions the passcode under "Set Content & Privacy Restrictions". You would choose a passcode different than the one used to unlock the phone – one that your child did not know.