As many posters have said, I don't think it's rude as long as you're fairly quick and organized about it. I would say 3 photos and 2 quick autograph signings is about the limit though. I do NOT appreciate it when either in a character line or at a meal, there is a group and they all want individual photos, then they want a group photo, then they want just the kids in the photo, then they have a bunch of autograph books, they also want their sweet petunia to dance with the character, they want to hold a "conversation" with them, etc. That is a bit much, I think. Even at the meals - it can REALLY drag out the time you're there waiting to see them all.
I think it's important to just think of others and all the other kids who want to see them, and then to choose accordingly. For us, we have 2 children. They do the autograph books, so I chose to just get one picture of them together with the character. A few times I got extra photos - walking up to the character in line, for example, or if one of them did something silly with a child at the meal. I actually got 3 copies made of each photo - 1 for each of their autograph books, and then 1 for our family scrapbook. No need to have the children separately for us.