The Children's Manual actually has a lot of information in it, and you probably would find it helpful in understanding what is, and perhaps what should be, going on with your kids. It covers childhood development and at least touches on an enormous wealth of knowledge, addressing physical development, including coordination and strength, body proportion (which affects center of mass and balancing movements), and development of motor capability. It also covers emotional development, cognitive development, and social, moral, and ethical development, as well as principles of "mutliple intelligence" and such. Lots of stuff! The better you understand these things, the better you'll understand your child--in skiing and everywhere else--and the more effectively you'll be able to help him or her learn.
That said, like all the current PSIA manuals, it is poorly written, despite the wealth of valuable information it contains. And because it covers so many areas, it does not go into much depth in any of them. Still, it will give you a great handle on some of the information that is out there, and if it stimulates your desire to learn more, that's a good thing too.
All in all, I think it's the best of the current manuals--although that is not necessarily much of an endorsement. I'm not aware of any other source of so much information on the subject, even as just an overview or survey course in child education. So I'll recommend it, even though I think it could be much better.