I'm writing this post without taking the comics, videogame, and theories into account.
An assumption is that the flamethrower still works.
At the end of the movie, Childs approaches McReady with his flamethrower. If Childs is a Thing, all he has to do is flame MacReady. That's it. Game Over.
A Childs-Thing doesn't have to worry about surviving a freeze. It will survive. Once MacReady is burned to a crisp, Childs-Thing has time to ready itself up for the rescuers. Before freezing, Childs-Thing can get rid of evidence or plant evidence or whatever. The rescuers can find a frozen Childs-Thing and assume it's a dead human.
If Childs is human, why did he walk out the compound earlier in the movie? MacReady told him that if Blair tries to get back without him, Nauls, and Garry to burn Blair. And that's what Childs says he saw, Blair outside by himself. He must have assumed Blair had killed the other three, and that's why he went out the compound, to burn Blair.
Yep, we know the ending was filmed so that we really wouldn't know, but I can't get over Childs not torching McReady. When Childs approaches MacReady, he's got the flamethrower pointed at him. Why keep him alive?
The flamethrower doesn't seem to be "on", but I don't think it would take too long to turn it on. This is something a Childs-Thing could have done before approaching MacReady.