Answering your questions:
1. How can the two kids sneak outside a highly secured area? Why the soldiers did not prevent the two kids from leaving?
Ans: The movie already told you from beginning, this is the first time kids are allowed to stay in the city. Therefore there is not yet any established procedures how to deal with kids. Kids know how to find loopholes.
2. "how those infected can differentiate between those who are infected and those who are not"?
Ans: those infected have not lost their senses.
"Why can't anybody who is not infected pretend to be infected so that they won't get bitten?"
Ans: Nobody dares to try.
3. "completely unbelievable how it breaks out again. yeah no guards, no security cameras, haha who can believe that? but guess thay had no other feasible idea to jumpstart the outbreak. "
"the plot has lots of holes (e.g. nobody guarded the infected wife) "
Ans: There is always lax in security, no 100% perfectly secure security system in the world because human always make mistakes. The Americans are too proud of themselves they thought everything is under control, the virus is already dead. Try ask yourself why got American soldiers die every day in Iraq if their security is so good?
4. "it is a bad film. dun watch it. no character development at all."
"And totally no character development at all......"
Ans: How characters develop is not the message they want you to know . If you want one, the black guy who flied the helicopter initially did not want to save anyone except his comrade. But in the end he felt remorse, and saved the two kids.
Movie review of 28 Weeks Later