1. The children must live in your home more than half the nights of the year (183 or more nights). 182 or fewer nights is not enough. In Turbotax, if they did live with you more than half, select "7 months". Turbotax thinks "6 months" means exactly half, and exactly half is not more than half.
2. You must have income earned from working, and not have too much unearned income. That means you or your spouse must be working a job, and not taking a pension or social security. If you are retired and not working, you don't have earned income even if you have taxable income.
3. Be sure to select "grandchild" from the type of dependent, not "someone else."
4. You don't have to provide more than half the child's support, that test is for a different type of dependent. Instead, the child must provide less than half their own support. It doesn't matter who pays the child;s support as long as it is not the child themself.
