Both answers are correct. You can do it at PC in the terminal or wait until after you board. For us the line in the terminal was long so we waited until we boarded.
As you walk in, check out if there's a line to the youth desk. If so, wait until you're onboard and go to the kid's club. If no line, go there. Easy peasy.
We did it in the cruise terminal. I'm not sure if I'd recommend it. It was a HUGE slow line. We missed our boarding call because we didn't want to get out of line after waiting so long. I mean, we obviously could still get on the boat, but we had little girl crying her eyes out afraid we wouldn't get on the boat. Not quite how I wanted to start our dream vacation.
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