You can't separate the vouchers for each child to have his own share of the account. If you are using one ID, all of the vouchers will be lumped into that one account. Maybe the honor system would work, but a better solution would be to keep a written tally of who spend how much.
Yes they can create their own playlists and sync their own content using one iTunes library and one Apple ID.
After the kids have set up and named their own playlists in iTunes.....
Connect the iPad to the computer and launch iTunes.
Click on the iPad name on the left side under devices.
Click on the Music Tab on the right.
Click on all of the albums or playlists that you want to sync to the device.
Click on the Sync Music Heading in order to select it
Click on Apply in the lower right corner of iTunes