First off my top concern is the health and comfort of my chickens, and I believe frontline is extremely effective while being very easy on the birds. It was recommended by a well known bantam show breeder and I know he wouldn't use anything dangerous on his birds.
Secondly it is used in...
The two active ingredients in Frontline ARE approved for use in food animals. The active ingredients are fipronil and (s)methroprene.
Fipronil is approved for use in cattle, sheep, and chickens: http://parasitipedia.net/index.php?option=com_content&view=article&id=2465&Itemid=2733
And...
True however Frontline is used on tens of millions of dogs/cats kept as companion animals. That means millions of people come in contact with Frontline every day when handling their pet, including babies petting dogs and putting their fingers in their mouths, people preparing food, etc...
I would give the birds a break for a while and not use any unnecessary treatments, you don't want to kill them with kindness.
But for reference, I buy the frontline spray for cats and dogs and spray once under each wing (where it can touch their skin, don't spray their whole body). I use it in...
If this all started the same day you doused them with DE then that is almost certainly the culprit. I don't use DE either, to prevent mites/lice I give them a spray of frontline under each wing every few months.
The shavings probably aren't the problem here but fyi there are different types...
How long have they been sick? If it has only been a short time it could be a temporary cold. If it has been going on for several days/weeks then it could be something more serious.