Use duct tape or other cloth tape to cover the holes into the couch.  It
shouldn't be able to be removed by the little ones, and should be out of
sight from the humans.  I had to replace mine several times when it started
to pull off, but overall, it kept my ferret out.  Pull one of the couch
cushions a couple of inches out from the back and let them sleep behind the
cushion.
[Posted in FML issue 1152]