iPhone 5s forwarding automatically to another phone in the household after iOS 8 is installed
The problem you are having is due to the Facetime feature "iPhone Cellular Calls" which allows you to "Use your iPhone cellular connection to make and receive calls on devices signed into your iCloud account when they are nearby and on Wi-Fi."  This is fine if all of the devices are your own so that you can answer the call on any of your iDevices but obviously not a good idea for multiple people.

Are all of the phones signed into the same [primary] iCloud account? Settings &gt; iCloud

If so, you have 2 options.


Log them out of the primary account and sign in using their own personal AppleID accounts.


If you were using a single iCloud account so everyone has access to a shared account for Music/Movie/App purchases, you should switch over to the new iOS 8 Family Sharing feature.

If you haven't created their own AppleID accounts because they were under 13 or you didn't want to include your credit card on their account profile, with the Family Sharing feature, you can create accounts for each child that is automatically part of Family Sharing and all purchases must be approved by you first. See this Apple KB article for more details.


Turn off the iPhone Cellular Calls option in the Settings &gt; Facetime screen.  This will stop the calls from going to multiple devices for now but in the long run, it would be best to set up multiple AppleIDs and start using the Family Sharing feature.


Tip: When you switch over to the Family Sharing, you can either use the old shared iCloud as your own (the Organizer) or create your own personal account. 
 When setting up the Family Sharing, one of the screens will ask if you want to share your purchases or use a different account.  Choose the different account option and use the old shared account.  This 2nd method will share all the previous purchases made under the shared account out to all family members but gives you the option to NOT share your own purchases.  This is helpful for parents to purchase items that the rest of the family shouldn't have access to.  

If you want to purchase something that should be shared, either sign into the App Store using the old shared account (just remember to sign out and back into your own account) or simply use one of your kids devices.

Hope this helps :)