Don't bring your own babysitter, the sitting is free til 10 pm then 6 $ an hour per child( on Princess) , so with two kids, it will cost you 24 $ till midnight. Even if you did that everynight it still is much cheaper then bringing someone!!( that adds to less than 150 dollars) .
A sitter under 18 is still your responsibility, so you have just added to your load, you will need to get all the permission to travel forms etc. done,. A sitter over 18( unless it is your mom,
) is not going to want ot sit in the cabin with the kids everynight.
We also cruised without our kids for our 10th, and we loved it. We also cruised without our kids for our 15th, so fun abd relaxing!!!! We still take a family vacation each year, but we deserve some time to ourselves. Plus we are smart enough to not tell our kids how great cruising is
!!! We have 3 kids, and live very far from port ( ten hours flight time) so it would be very exspensive and hard on the kids to bring them( ok we made up the part about it being hard on them,...... it would be hard on us!!
Good luck and enjoy your trip.