I think the very farthest I'd go would be something for the kids to have fun with during the cruise itself, like the glow stick or glow necklace idea. But truly, it's truly not necessary.
I had a very wonderful experience sharing an IllumiNations cruise in February. I was the one with the reservation. It was just my wife and me and our 2 kids (ages 8 and 6). The trip was a celebration of my wife's 40th birthday, and coincidentally our son's 9th (it was a few weeks before both).
Maybe a month before the trip, I decided that it might be cool to sprinkle some Pixie dust and share the cruise with another family; the boat fit 10 people. I posted in the cruise share forum here on the DIS, but I was very picky about whom I selected ... I wanted a family similar to ours. I didn't want a couple with no kids, because it would probably be difficult to hang with them while we'd need to keep tending to ours. And we didn't want a couple with teens because the kids wouldn't be able to easily socialize with each other.
Oh, and the cruise was a surprise for my whole family.
Well, the DIS being as great as it is, I found a man who also wanted to surprise his wife, and they were taking the trip to WDW as a celebration of his wife's 30th birthday and coincidentally his son's 8th; he also had another son who was 5. It seemed like the perfect fit I was looking for.
We communicated via e-mail to agree on things. I hadn't been interested in doing anything special ... I thought the cruise was special enough. But he wanted to order the $25 Celebration package which included decorations and a Happy Birthday banner. He also wanted to order a birthday cake ... we had to agree on the size and type, and we ended up with a pretty simple one that cost a bit over $42 (and rather than try to cram 4 names on one cake, I came up with the idea of the bakery writing "Happy Birthdays" on it). Private Dining at the Yacht Club doesn't come inexpensively!
Anyway, we exchanged cellphone numbers and the name of the resort each was staying at. He texted me a few hours before the cruise to make sure everything was still cool. He told me he was wearing a green hat, so I'd know how to recognize him at the dock. I texted back that it made me feel like a spy, to be surprising my family and at the same time looking for a man with a green hat.
Anyway, all 8 of us had a wonderful time. The kids were at the front of the boat with the food, and the grown ups were at the back of the boat enjoying the quiet of not having to dote on our kids every second, as the 4 of them had a lot of fun together.
As for the 4 grown ups, we couldn't have come from more different American backgrounds ... two liberal Northern Jews, a physician and a psychotherapist, and two conservative Southern Christians, a lawyer and a stay-at-home mom ... and we couldn't have had a nicer time.
And I felt very good about opening up this opportunity for their family. It wasn't about sharing the expense of the cruise; I had been perfectly happy to pay the full price when I made the reservation, since at the time I'd never even heard of sharing a cruise.
And if I'm ever in Alabama, I have a place to stay!
They had already booked the cruise, and were looking for more people to share the cost, so it will be them, my family, and another family. We leave in 9 days, and now I'm wondering if maybe I should get them some kind of little thank you gift or something? I've never shared a cruise, and don't know the etiquette. I don't want to show up empty-handed, and the other family gets them something, how embarrassing! And if a thank you gift is expected, does anyone have any ideas? They have a couple of boys, I think they are 6 and 8, so maybe a little something for the kids? I don't know. 
