I have dealth with many kids, and in my opinion, most complaints that revolve around kids being mistreated is because, the parent always feels that their kid is right and is an angel. Like everything else, there is two sides to a story, and most likely, the kid is at fault. (not always).
I have two kids, and they have never had any issues with bartenders on a Carnival Ship. The bottom line is this. The bartender has 10 customers, that is waiting for a drink. This is a cruise ship, those customers are not leaving. Each one will be served, and each one will have a tip. The kid with the soda, is just another customer and he/she is not going to loose a tip, because of it. If anything, if the customer sees that they are working the bar, paying attention to what is going on, and squeezing in those kids, they just might actually get a bigger tip for hustling, and being impressive.
Think about that.