R..
Is the basement door right off the main rooms, or do you have to go through a hallway? You could block off the entire hall if it's not on the main room... Oor, you could get one of those baby corrals (looks like a bunch of interlocking baby gates) to set up a safe zone where you can stick her when you go downstairs. Then you know she isn't getting into anything when you aren't able to immediately supervise her. :)
Also... I would closely supervise her, and 'teach' her how to back down the stairs safely. That way, if she ever DOES get access to the stairs, she will know what to do instead of crawling and falling facefirst.