I was able to do this online on Allure in 2014, as well as recently on Harmony. Since pp noted it was a last minute trip, there could have been some online issues specific to that.
When my kids were younger I was able to do it online also. They always verify at check in whether I want the kids to be able to charge. Now that they;re teens I give them a limit and they know not to go over it.
I know on one trip one of the kids were able to use their card in the arcade and two weren't. Even then though they have to put the seapass in a machine and have cash added to their seapass then they can use their seapass for games. If they try to just put their card in the games it won't work.
We’ve done several RCL cruises and I love Disney with a passion but I can’t bring myself to spend the extra money for a Disney Cruise. We had an amazing time on our RC cruises and I can’t imagine it getting any better and paying thousands more just to have a Disney theme (even though I do love it) just doesn’t seem worth it. So we stick with RC and save that additional money to spend at the parks!