Unfortunately because of their ages the law was different and your children would not qualify for automatic citizenship. In order for them to qualify you either had to be separated from your wife and have legal custody of your children, your wife must have died or she must have naturalized before they turned 18 like you did. Therefore, they are not automatic citizens and cannot file the N-600. If they want to become citizens, they each have to file the N-400 applications and apply for citizenship on their own. Keep in mind that the price for citizenship will increase after December 23 2016. I'm sorry that I do not have better news. I wish I did but I hope you understand that I do not make the law and I cannot change it.
Good luck to you and I hope this answer has helped you!
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