The book Adam's Tongue by Derek Bickerton chronicles one view of how language evolved in humans and in doing so, evolved humans along with it. It's pretty fascinating.

The book explains how our brains wouldn't really have the "thoughts" you speak of without having had language in the first place. I'm not sure precisely how this fits in to children who can't speak and who were never taught to sign, but my guess is it would introduce considerable mental stumbling blocks.