Note that the "special rules for children of divorced or separated parents" described in IRS publication 501, do not apply once a child turns 18, or is emancipated under state law (in some states this may a different age than 18. The taxpayer needs to check the laws of their state.)
That means that the rules about transferring a dependent by form 8332, no longer apply. It does not mean you can ignore a divorce order, but it may mean the divorce order is non-enforceable under federal tax law.
In your specific case, assuming that child #1 lives with you more than half the year, but the court ordered that your ex can claim child #1, then in previous years, what should have been happening is that you would indicate on your tax program that the child lives with you more than half the year, and you are releasing the dependent to your ex by giving your ex a signed form 8332, or because your ex has a qualifying court order dated 2008 or earlier that they will attach to their tax return. The child still qualifies you for head of household or EIC, because those benefits always stay with the the parent where the child actually lives and can't be waived or transferred.
Then your spouse would indicate that the child lived with them less than half the year, and they are claiming the child because of a pre-2009 order or a form 8332. They get the dependent exemption and tax credit but not HOH or EIC, and they have to mail a copy of the court order or the original form 8332 to the IRS after e-filing.
If you have not been doing it this way, you might have been depriving yourself of tax benefits, and your ex may have been claiming unallowable benefits.
Now, once the child turns 18, those rules no longer apply. If you gave your ex a form 8332 to claim the child, it would be invalid, and your ex can't claim the child by attaching a copy of the divorce order to their tax return. The normal rules now apply.
That means you can claim a child who lives with you more than half the year, provided that they are age 18 or less, or if they are over 18 but under 24, they are a full time student. "full time student" means full time as defined by the school, for at least one day of 5 months. So, a graduated high school student is considered a full time student for dependent purposes, even if the graduated in May and did not go to college in the fall.
Since your child is 18 as of 12/31/2020, you don't need to worry about the full time student rule. You can claim your child as a dependent if they lived with you more than half the year, and they did not earn more than half their own support.
This is not to say you should do anything to violate your court order, but parts of that order may be invalid as far as the IRS is concerned. The IRS will not object to you claiming the child even if your ex also tries to claim the child, although the IRS may send letters to both parents asking for an explanation. Your ex could complain to the court, but the IRS rules are clear that, once the child is emancipated, the ex can't claim the child unless the child lives with the ex.
