You cannot cover medical expenses for children not claimed as dependents on your tax return. However, if you and a former spouse were legally divorced or separated at the end of the calendar year or lived apart during the last six months of the calendar year, your child is then treated as a dependent of both you and your former spouse, even if your child’s tax exemption is claimed by your former spouse. In this situation, you could use your HSA to pay for your child’s eligible healthcare expenses.
It is important to note that if you use your HSA to pay for your child’s eligible healthcare expenses, your former spouse can’t use his or her HSA to pay for the same expenses.
For more information, please refer to IRS publication 969.