Question 1) I want to know when should I submit my Express entry profile (I started it two days back). Should I wait for all the experience letters and PCC before submitting?
Since you are in the US and FBI has recently started eDO, the US PCC should not take more than a couple of days. The Indian PCC would be handled by the consulate and again should not take too long. It depends on how much time your former employers take in furnishing the experience letters. Even if they take time or decline to provide them, you would have the option to explain this to Canadian Immigration while submitting your documents.
I noticed in another thread that your score is 453. The recent cutoffs have been in the 440s so in my opinion, you should apply for EE right away. The next cutoff is expected around March 7 so if you apply now, you might be able to qualify in that pool. After you receive the ITA, you get 3 months to submit all documents. So you have that window to work on the PCC and experience letters as well. Also, you will be able to schedule the medical tests only after you get the ITA.
Question 2) I have work experience letters from my first Job which covers three years from 2008-2011. Is that sufficient? Or do you advice to get all the experience letters uptil now?
I am not certain about this but I think you should provide experience letters for all the positions which you have mentioned in your EE profile. Only if the employer declines from providing the experience letter, you would be able to mention about that in a letter of explanation.
Question 3) My kids are born in US (< 5 years), so is there any documentation needed for them?
No idea about this.