Selective herbicide. Just be sure to maintain a proper spray rate so that the seedlings don't get too much chemical on them and die anyways.

Check out the labels on some of these suggested herbicides to see if they kill trees or not. I've never sprayed around seedlings that couldn't be killed before, so I'm not sure exactly what chemicals will and won't kill them. If you do go through commercial sprayers they should be able to tell you what chemical you need. Just be aware that if you hire professional sprayers it might cost you a small chunk of money. My company charges $50 per hour per 4 wheeler (we run 4 machines), plus chemical cost, plus mileage. And we are more affordable than much of the competition.

http://www.weedalert.com/cgi-bin/recs_view.cgi?weed=35