I don't think everything needs to be made fair between kids at all times. If the ones birthday fell during your trip, I see nothing wrong with a special meal and singing happy birthday or opening a gift. But, I think it could lead to hurt feelings if you go above and beyond for just one of the kids. If it's a family vacation, they should both be in on the fun. I think it's unreasonable to expect a six year-old to just suck it up and be happy they're in Disney.