Workflow issue

One thing I hardly can get - I want my co-workers to be able to edit theri ads. But I have all edits to be reviewed by moderators - like if one wants to extend or cut campaign, he should be able to do that, but that change should be "pending". Or one should be able to change image or link, but the old version should be displayed until moderated.

Possible? How? And if not - how to start making that happen?