Just use her current sim card when the phone arrives.
The iPhone 11 is not 5g capable, so no need for a new sim.
However, unless you are buying the new phone outright, O2 will text the current phone during the upgrade process, so it won't be a surprise if your daughter sees the text messages.
You'll need access to the phone to receive the code sent when you start the upgrade at the very least.
There may also be further texts with order update info, just so that you are aware.
Track usage, upgrade and get support with the My O2 appDownload My O2