We've taken the Nickelodeon tour with our kids a couple of times.
During the tour, they select 1 or 2 kids to taste test slime (or
maybe it's gak, who over the age of 12 can keep these things
straight

) Then at the end of the tour during Game Lab, one
child is "chosen" to be slimed. It's really just a question of right
place, right time. My oldest son was chosen to do the taste test
and then refused to eat it! Well, I got some nice pictures anyway.
Game Lab, by the way is a mock Nickelodeon game show--think
Double Dare or Wild and Crazy Kids. All kids sit on one side and
all adults on the other and they play two teams against each
other. Quite a few kids get chosen to compete in the various
games--not to mention some of the adults. The kids think it's
great, even if they're not chosen to be the slimed one. I'm sure
your kids will enjoy it.