I don't think either would be a great idea for children, no matter how well behaved they may be. I've been on Crystal and Seabourn, and found very few, if any, on board their ships. Seabourn, with their very small ships, don't cater to families at all, and they don't have much for them to do. Dinners last nearly 3 hours, so if your child can handle that, then go for it, but be aware she will most likely be the only child on board. Seabourn is also a very formal cruise line, as is Crystal, with dress codes being enforced in the evening. I'm not saying your child won't like to dress up, but both those cruise lines are the most formal of any cruise line. Seabourn has no children's program or even a room for kids to play in. As for Crystal, again, they don't much cater to families. I've done a number of Crystal cruises in the Med and Baltic, and I've not seen many children on board. They need a minimum of children before they have any kind of kids program and even then, they don't have anything like you find on RCCI, Princess or Celebrity. A better choice might be Celebrity. They offer a very good children's program and there are a lot of kids on board in the summer. Celebrity is a more upscale cruise line, with excellent food and service and they serve a very wide range of ages. Your daughter would be among at least 200 or more children. But, in the end, it's your decision. As I said, neither cruise line caters to children, and the average age of both cruise lines is well over 60, even in the summer. If your daughter won't be bored with no organized activities, a very upscale and formal experience, and hanging around with you for most of the time, then go for either cruise line.