I know this is an old post (before someone jumps on that fact). ;-)
I just wanted to add that sealers and other such products are petroleum-based and give off very toxic fumes, much more so than any natural wood oil could do!
If you're very concerned about the cedar oils, you can always get some dishsoap and scrub both sides of the boards (or, better yet, just do the box after you've built it), then rinse and allow to dry. That will remove much of the surface oil; any oils still in the wood will come to the surface over time, but these should be far diminished as they will dissipate as they're being released, instead of all being there at one time.
I did not know that cedar shavings were from a different species, and had worried about using Western red cedar for my brooding box, but after reading about it, I've decided there's very little, if any, danger to the chicks, providing there is air-flow. Stagnant air could hold fumes in, but even a wire-mesh top should be more than adequate for air-exchange. Without a low-pressure system holding air down above the box (unlikely unless your barn is the size of a Boeing hangar, lol), it should have no problem dissipating oils as they rise to the surfaces of the wood.
In fact, to be honest, cedar may actually be one of the better woods to use, as it's naturally bug-and-rot resistant. And we all know that chicks aren't exactly the cleanest and driest creatures.
On the topic of fumes, some may know that linoleum gives off a bit of an odor too, being another petroleum product, but with enough ventilation that's actually a fine material to use for a floor in a coop. I laid it in both of my coops, and the girls are just fine -- plus, it makes cleaning the coop a breeze. I hook the handles of a black trash bag from nails I drove into the rim joist, open my "human-access-cleaning-door", and just slide everything toward me with a rake. It goes in the bag, I take the bag to the compost pile, dump it, and repeat. Takes about 5 minutes to clean the whole coop.
I suppose you could use actual ceramic or terra-cotta tiles, but I don't think it's necessary.
