Why can my child access the Screen Time menu on her phone when I have content restrictions enabled?

You cannot hide, it, but if you enable a password for Screen time she would not be able to change anything unless she knows the password.

If you have not enabled a password, then Screen time settings and available to modify.

click here ➜ Lock Screen Time on your iPhone or iPad - Apple Support

You need to lock down Screen Time on her device, not on yours.

If using family sharing you select her account from the Family option in Settings, and then screen time and within those settings you can lock it down and add a passcode. It will request you set up a passcode for it when you set up things like Downtime. This passcode should not be known to your child.

You should also not associate the passcode to her Apple account for recovery. You need to use your Apple account, and she should not know your account's password either.

The entry in her device for Screen Time will be still there in Settings, and cannot be hidden and can still be accessed and she can see all settings within it, but without the passcode, no changes can be made. Anytime a change is attempted, the Screen Time passcode should be required.