Need your help.

hello Wang Chun,

Your sentences are not "bad" in themselves, but they do need a bit of polishing.

1 - "Disadvantages of sports are less than advantages of sports for children in my book. "

You could reput it this way:

In my book, there are many more advantages than disadvantages resulting from the practice of sports for children .

to my eyes, "practicing" is better than "doing" for sports;
emphase should be put on the source of your statement, your book;
the positive form, unless you attack another argument maybe, is usually better;

although my sentence also could use some more polishing and thinking, I think it shows a bit the kind of remodeling the two other could use.

Your second sentence is also a very direct statement. Can you support or defend it?
The third one is a bit like the first one. It just needs a little reorganisation... ex:

A common phenomenon, among modern young students, is that they like to eat saturated-fat-food such as fast food.

I typed this very fast, so please don't take it as absolute truth! But that's the way a 2 minutes post is!

good luck