therefore time on this activity is really time wasting
in order to catch the eyes of the children,majority of the games use war, violence, bloody, or sexual subjects to lure the students.
You should separate this sentence into two sentences, and in fact, these sentences should be put in different paragraphs. Because in the 2nd paragraph, you are talking about how computer game steals time from children. Thus, in the next paragraph, you may introduce the violence conveyed to children by computer game. I believe that there are many examples of children or teenages that applied the violence from computer games to their real life such as gun shooting, raping, etc.
Admittedly, there are a few students paying considerable time on the computer games playing finally choose computer games related jobs as their career.
Admittedly, ...is used at the beginning of the sentence to introduce a concession or recognition that something is true or is the case.
For example, "admittedly, the salary was not wonderful, but the duties were light."
So, you may say that
Admittedly, there were students who eventually became game designers, but despite the time that they lost, only a few of them went for this job.