I did it this month from the Dream.
There was one under-12, and the CM did not give the child a bib (though I think she told the the parent she would would get the child one later, on the down-low, but not FOR the 5K). And...I passed them later, and neither of them looked happy. The child was walking and dawdling, the parent was highly annoyed and threatening to just go back, etc. It made me very glad I had not taking my highly active 8 year old. Well, it was moot because he refused to wake up, LOL, but if he had woken I'm glad I left him with his dad in the stateroom. (they were able to be first to meet Jack Sparrow that way!)
The Navigator you get before the Castaway day will tell you when and where to meet. Meet there on that day. Have your keycard and your photo ID (though for my 5K they were being really relaxed with those photo ID rules, since it seems almost everyone forgot their ID), and know there are no bathroom breaks. They walked us off the ship and to the starting line.
I did it for the experience and the bib number, and was incredibly happy to get a medal at the end. And it allowed me to relax allllll day, knowing I had gotten that exercise in early that morning.