"Tube Audio" <murphysf@hotmail.com>:

They love it. They are playing! Leave it alone.

Rub a little more catnip, but on different spots. Then they will have
different locations to go to. Otherwise leave it alone, something they
are enjoying is not a problem.

it could also be that the cat is marking the area which all cats do when
they rub their face on a location. it is their way of saying it is mine

if their is no blood and bloody screams; other than get the heck of me.
They are  playing.

these are also ferals cats  there is a dominant social order that will
develop.  the kitten might be establishing dominance but unless there is
blood or the kitten is preventing the others from eating  than there is no
problem.

it also might not be catnip it might just be an odor.  One of my cats gets a
whiff of a certain cleaning solution and he gets aggressive.

if you must do this  you can take bleach and sanitize the area.
but make sure you wash the condo after to get the bleach smell off or they
will not use it.

Response from "Tube Audio" <murphysf@hotmail.com>:

Everyone already said it perfectly.  The thing is you mentioned 2
fundamental things:

1.  Feral

Even non-feral cats have a pecking order, feral cats I would assume
adhere to it much more strictly especially when they have grown up
together.

2.  Catnip

Cats adore catnip assuming:

1.  They are old enough to be affected by it.
2.  They are affected by it at all.

I really wouldn't worry about this too much.  However, I would like
to tell you this -- something to keep in mind.  Kittens are a
handful.  We have only had our Gabby for almost 2 months or so now I
guess and it has been an interesting time for all.  There are times
we have wanted to boot him out the door then he does something that
melts our hearts.

So basically... unless you see an ear come flying off I really
wouldn't worry about their play.  Although you should be aware of
their sounds.  Low guttural sounds or long drawn out low mewing and
loud sharp short yowls are NOT good.  The play MAY be getting too
rough.  Just be there to help if things get RIDICULOUSLY out of hand
-- otherwise remember, a little domination is key to a structure.

$0.02 from a new cat's slave's perspective.