This happens when you share the same iCloud account; any synced data is merged. You can share the same iTunes account but you need separate accounts with separate IDs for other services such as iCloud, FaceTime, iMessage, Find My Friends, etc.
One of the devices can keep the current account; on the one that will be moving go to Settings>iCloud and turn all data that is syncing with iCloud (contacts, calendars, etc.) to Off. When prompted choose to keep the data on the device. (If you are syncing iWork documents with iCloud, also open your iWork apps and turn off iCloud syncing and choose to keep the documents on your phone.) After everything is turned off, scroll to the bottom and tap Delete Account. (This will only delete the account from this device, not from iCloud. The other device will not be effected by this.) Next, set up a new iCloud account on this device using a different Apple ID and turn iCloud data syncing for contacts, etc. back to On. When prompted, choose Merge. This will upload the data to this new account. At this point you will have two different iCloud accounts, one for each device, 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 your kids' contacts from your account, and vice versa). These changes will be reflected on each device; when finished you will have individual iCloud accounts with just your own unmerged data on them.
If you need to change the ID for other services to keep them separate, for iMessage to to Settings>Messages>Send & Receive, tap the ID shown, sign out, then sign back in using a different ID. For FaceTime, do the same thing in Settings>FaceTime.