To file under the head of household status, you must pay more than half the cost of your household. A household is not necessarily everyone living under the same roof. For example, if you had a three bedroom apartment and sublet one bedroom to a girlfriend and her daughter, but you had separate food and separate living arrangements and separate social activities and separate friends, then you might have two separate households.
If you share meals and social activities with your parents (the child’s grandparents), and especially if you don’t pay rent, then it is very likely that the IRS will see your living arrangements as being one household. If you don’t pay more than half the total living expenses of that household (including rent or mortgage, food, insurance, utilities and maintenance) then you do not pay more than half the household expenses and are not qualified to file under the head of household status.
To file as head of household, you would have to be able to convince the IRS that you and your child are a separate household from your parents, and that you paid more than half of the entire cost of maintaining your separate household.
