Your kids are too old for you to get the child tax credit. You only get the $500 credit for other dependents for kids older than 16. You will be able to claim them as dependents if they are full-time students until the year they turn 24. So, yes, you will be able to claim the 18 year old.. and file as HOH. The parent who can claim a full-time student as a dependent is the one who gets to use the American Opportunity Tax Credit.
https://ttlc.intuit.com/questions/3262983-who-is-eligible-to-take-the-american-opportunity-tax-credi...
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