It sounds like you're talking about adult non-fiction bestsellers. I don't think that has much to do with the reading habits of nine-year-olds. There are plenty of 80-150 page books being published for this age group.
I love an endless series of giant fantasy novels, but I've read so so many SF short stories that really stick with you and make you think. As long as you're not going back to the old pulps, any decent collection is probably really good.