I've cruised w/3 kids 1 boy 2 girls all related though they had an inside across from my room. Took turns changing in the bathroom it was fine. but adjoining is an idea. No problems w/them doing their own thing onboard. they skated, ate, arcade, email and such they had their sea passes around neck the whole time. I put $$ limits on them for the Cruise. They had to check in often most of the time I was near watching or reading anyway.
For mine if they were caught by anyone misbehaving I was their new best friend the WHOLE trip..lets just say I had NO problems the whole Cruise
Passport issue I have had this problem too but for the Canadian Passport anyone over 16 is an adult passport thus they sign there own. But before that age I had my Lawyer draw up a letter for the other parent to sign permission to allow the minor child to be able to leave the Province or Country. Make sure to have it notarized at the Lawyers at that time also, I then went to the Goverment with that letter and copy of custody papers and it all worked out fine. Since you have time to do this I would suggest this. HTH