The question, not addressed by the other answer, is does your child qualify. You only said previous relationship.
The child qualifies you to file HOH if the child is:
1. Your biological child
2. Your stepchild (even if you divorced, the child still qualifies you)
3. Placed in your home by the state or a state-approved foster care agency.
4. Your adopted child.
However, if the child is not one of these types of relationships, and you just ended up caring for them because families are messy sometimes (for example, the biological child of a former partner whom you never married), they are not a qualifying child for HOH.
You weren't specific, so I just wanted to put that out there.
