We're moving into a basement...

I have a question, but have looked at some of the flaming that seems to go
on here, so I am trying to quell any issues with a little background.

My wife, myself and our two cats are moving into my parent's house.
However, we're moving into the basement. It's a partially finished basement
that has heat and A/C. We will be living and sleeping down there, coming up
only to eat and use the bathroom. We're going to install a door to the
laundry room to keep them from the sump pump and washer/dryer.

Now on to the issue.

My parents are not "cat people" and would prefer that the cats stay in the
basement with us. We have no issues with that, but we are trying to figure
out the best way to keep them there. My thought is to try and have some
"two door" system...meaning they would have to get past two doors to get
upstairs. I thought of this because I know how they get sometimes when in a
smaller room and bolt past when a door is opened. They did have a door at
the top of the stairs that was removed while remodeling. I believe the door
will be going back on. So our thought is to find or construct something to
put at the bottom of the stairs that we could easily move...something
w/wheels perhaps? The stairs have walls on both sides, so I guess you could
say it's less of a basement and more of a "lower level" of what would be a
two level house.

Any thoughts? Please, please do not flame or ask us to get rid of the pets.
We are just trying to find a happy compromise.