Escape Artists

I wrote here about how C had learned to escape the pack n play and his crib. 
He had not done it since that night we came home and he was totally exhausted, so I thought maybe I had dodged that one for awhile longer.
Thursday I heard the kids talking after their naps.  I normally don't go get them up until I hear at least 2 of them, or maybe all 3.  They are happy talking and playing, and I am happy with them in there.  So Thursday I finally walked in, and I walked in to L straddling the crib rail, and C lying flat across the crib rail.  In my sternest voice I say "No, no!  That is dangerous!  We do NOT crawl out of our cribs!".  C was unphased but got down, but my sensitive little L looked at me and burst into tears. I hoped maybe that had done the trick. 
Friday morning, I heard the kids talking again, and finally got out of bed to go get their milk.  All of a sudden I hear footsteps, and I am just hoping it is Z...but no.  C is just grinning up at me.  So I go back to their room, and see N still in his crib, but no L.  "N, where is L?".  N just pointed under the changing table, where I see L grinning and hoarding pacifiers. 
Friday at naptime, I was worried.  I tucked them in, then shut the door to peek in.  I watched L get out (his is the only bed I can see from the door), then walked back in and find C had escaped too.  I grabbed each of them, gave them a firm swat on the bottom, and said NO!  Well that sent all three of them into screaming fits.  Over the course of the next 40 minutes they probably got out 4-5 more times, and the screaming didn't stop.  Finally I just sat in there with them, not looking at them, but watching them.  When L tried to get out I would tell him no firmly again.  They all settled down eventually with me sitting in there.  L fell asleep, and C and N were calmed down about 30 minutes after I went in there (an hour and 10 minutes after naptime "started").  I got up to leave and C and N cried for a minute, but then went to sleep. 
Since then no one has tried to escape again (and now we have had 7 opportunities).  Hoping that put a bit of fear into them, and that maybe they won't try again for awhile longer?  We'll just have to wait and see....