iCloud+

Have you verified that the invitation you sent to your daughter was accepted? After you send the invitation, you can use your device to check the status of the invited person.

You can check this easily. Apple's support document Set Up Family Sharing says:

On your iPhone, iPad, or iPod touch

Go to Settings, tap your name, then tap Family Sharing.

Then select that person's name to see the status of the invitation. If you need to resend the invitation, select Resend Invitation.

On your Mac

Choose Apple menu  > System Preferences, then click Family Sharing. If you're using macOS Mojave or earlier, choose Apple menu  > System Preferences > iCloud, then click Manage Family.

Then select that person's name to see the status of the invitation. If you need to resend the invitation, select Resend Invitation.

These instructions might work on your daughter's device, although the first sentence does say that the free 5 GB plan switches automatically, so it's long shot. If it doesn't work sign her out of iCloud and then back in. This might reset the connection to the plan and resolve this problem.

Just one other possibility - you didn't mention if you have just organised the new plan and/or invitations and are still working on it or if this has been a problem for some time. I found when I set it up some years ago the the statement of storage plan did not update until I quit Settings on my daughter's phone and then reopened it. I don't just mean push it up out of the way but close it completely. When it opened again it was displaying the newly accepted plan and the correct amount of space. Might be worth a try.

Apple has the following instructions at the linked page:

Switch from your iCloud+ plan to your family plan

When you share iCloud+, any family members using the free 5GB plan are added to your family plan automatically.

If a family member is already paying for their own iCloud+ plan, they can choose to switch to your plan or keep their own and still remain part of the family. When they switch to the shared family plan, they get a refund for the remainder of their personal plan. They can't keep their current plan and use the shared family plan at the same time.

On your iPhone, iPad, or iPod touch 

1. Go to Settings > [your name].
2. Tap Family Sharing, then tap iCloud Storage.
3. Tap Use Family Storage. 

On your Mac

1. Choose Apple menu  > System Preferences, then click Family Sharing.  
2. Click iCloud Storage.
3. Click Use Family Storage. 

If you're using macOS Mojave or earlier:

1. Choose Apple menu  > System Preferences, then click iCloud.
2. Click Manage Family. 
3. Go to My Apps & Services. 
4. Click Use Family Storage.