I know this is debated often, but I do not believe Childs is the thing because he is still wearing his earring in the same ear. The thing cannot copy metal.
McReady is the thing.
Earlier in the movie, the team decides to prepare their own food and eat out of cans because this would spread the thing. McReady shares his opened drink with Childs, then chuckles when he drinks it because McReady knows he just won.
