Settings > iTunes and App Store > (Automatic Dowloads) Apps > Off
Settings > Messages > Recieved At > Uncheck any number/email that doesn't belong to your device
The former should stop the apps from downloading to your device, the latter should stop your messages from going to thier device(s). I suggest that you adjust these settings on their device(s) as well.
You can create a new Apple ID for the kids here > https://appleid.apple.com/cgi-bin/WebObjects/MyAppleId.woa/
You can then follow the former example above to enter the new Apple ID on the device, just tap Apple ID and Sign Out.
Beware that apps and music do not transfer between Apple ID's, they are forever attached to the Apple ID used to download them. I would try the settings first before the Apple ID change. That may save you some headache