What you should do is get them their own Apple ID’s and then set up Family Sharing. You can all share purchased content. The Family Organizer agrees to pay for all purchases with a debit or credit card. If the children are under age 13 you can set Apple ID’s up for them yourself.
This is not that difficult to do but it takes a little time to get it all set up. Take a look at Eric Root very helpful response here.
Everyone needs to be on their own (free) Apple ID. Then sign out of all devices and sign back in with the separate IDs on their own devices.
If I get them new Apple ids won’t that mess up billing and other issues?
Michael provided the answer.
Use family sharing for purchased content sharing. An AppleID is meant to be a single persons unique personal identifier for all Apple services (iCloud, FaceTime, iMessage, the App, iTunes, iBook and Mac stores, these forums,...). They were never intended to be used by a multiple people, and with the increasing number of features using iCloud to sync data and content across devices, sharing an AppleID will simply cause ever more headaches and problems since the data is not separated by user (the system sees one user = the AppleID signed in).
iMessage is merging peoples messages since ios12
