First, depending on how much you know and interact with the other co-workers, I would just keep cool. I wouldn't make a huge deal out of this, or rub it in anyone's faces.
If you choose to approach them about this, do this individually, and start off with a simple greeting, some small talk, something like that to lighten up the mood and keep it casual.
Then I would go into asking how they feel about applying for that position, and the key here is to listen to them, see what they would do in that position, what they could improve, etc. Mention some things you agree with, so they can be put at ease that if you get the job, you'll still communicate with them and that you won't be harsh on them.
After that, maybe briefly mention that you are also applying and that if you don't get the job, mention that there won't be any hard feelings because you know they will do a good job too based on their experience level. Maybe add a joke in there to lighten things up like
Hey, if you get it, you can do [something funny, like being responsible for buying everyone lunch]... ah, just kidding.
(If they are not the kind of person to make light of that joke, I don't recommend it.)
At the end, say something like "good luck" or "We'll see what happens" "Whatever happens, we'll still see eye to eye", etc
This whole dialogue is meant to put the co-worker at ease to help them see you will indeed be fair and friendly no matter what position you're in or they're in.