How do we separate family's iPhones and iPads into separate devices with their own AppleID?

First, read the following - Family Sharing - Apple

In there you will learn how to set up Family Sharing, setting up Apple IDs for children under 13, turning on Ask to Buy, etc. Just make sure that you do not sign anyone into a new Apple ID before you save the information you want on each phone. Also, just to be sure you may want to run a backup on each phone you want to change the Apple ID just in case something goes wrong so you can restore it if necessary.

Go to Settings then select the name at the top of the page then select iCloud. If you want the existing contacts to remain then deselect Contacts and you will get a message asking if you want to keep them on the phone or delete them. Select to keep them on the iPhone. You should be able to do this for Contacts and Calendars.

Go to Settings > Photos and select the option to Download and Keep Originals. Once all photos have been downloaded to the device then turn off iCloud Photos.

If there are any notes they want to keep you need to make sure they are not in an iCloud folder but listed under On My Phone.

Once you have done the above, go to Settings, select the name at the top of the screen, scroll to the bottom and select Sign Out. Now, sign in using the new Apple ID for the device. Once that is done you can turn on the option to keep Contacts and Calendars on iCloud and turn on iCloud Photos again.

After everyone has their own Apple ID, invite them all to Family Sharing and turn on Ask to Buy if you desire.

You have to transfer the important data manually and then just create new fresh Apple IDs for everyone and sync the data they need. There's no way to extract an exact piece of data and transfer to another Apple ID.

Just sign out of the old Apple ID and sign into the new one. What data are you wanting to merge?