If you mean that you want to switch their phones to thier own iCloud accounts, on their phone's go to Settings>iCloud and turn all data they are syncing with iCloud (contacts, calendars, etc.) to Off. When prompted choose to keep the data on the iPhone. After everything is turned off, scroll to the bottom and tap Delete Account. (This will only delete the account from their phones, not from iCloud. Your phone will not be effected by this.) Next, set up a new iCloud account on each phone using different Apple IDs and turn iCloud syncing for contacts, etc. back to On. When prompted, choose Merge. This will upload the data to the new accounts. At this point you will have three different iCloud accounts, one for each phone, with identical data on them.
You will then have to go to icloud.com on your computer and sign into each iCloud account separately and manually delete the unwanted data (such as deleting their contacts from your account, and vice versa). These changes will be reflected on each phone. When finished you will have individual iCloud accounts with just your own data on them.
You can continue to share the same Apple ID for purchasing if your prefer; it does not have to be the same as the one used for iCloud. Also, if you want to change the ID used for other services you have to use a different process. To change the ID used for iMessage, for example, go to Settings>Messages>Send & Receive, tap the ID at the top, sign out, then sign back in with the different ID. For FaceTime, go to Settings>FaceTime and do the same. For purchasing, go to Settings>iTunes & App Stores, tap the ID, sign out, sign back in with the other ID.