I'm a H1B visa holder and my kids are staying with me for the whole year and now. When I file tax, it shows my children are NOT dependent. May I know why?

[KarenJ]

You can only claim your dependents if you and your children are considered resident aliens in the US.  

You and your children need to pass the substantial presence test:

 You need to have 183 days in the US using the following calculation, count all the days of the current year (2017)you have been in the US, count 1/3 of the days you were in the US in 2016 and count 1/6 of the days you were in the US in 2015.  If the days add up to 183 US days or more, then you and your children are a US resident for tax purposes.  

Please see attached for IRS Publication 519: Publication 519 US Tax Guide for Aliens