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