Yes, if your dependent child had enough income that he is required to file a tax return, then his income must be included in your household income to determine the allowable premium tax credit on marketplace insurance. Generally, a dependent is required to file if they exceed any of the limits on this table.
The household income determines how much premium tax credit you are eligible for. It does not matter if that particular member of the household is covered by the insurance. It would be the same if your spouse had their own coverage through their work. Their income would still be used to calculate your allowable premium tax credit even if they are not on your marketplace insurance policy.
