The employer is supposed to do that.
Under federal law, the employer is not allowed to provide tax-free health care for your DP unless your DP is also your tax dependent. Normally this means that healthcare premiums paid by you (employee share) for the DP are after-tax, not pre-tax, and the employer share of premiums is taxable income to you. The employer should include this on your pay stubs and W-2 automatically. You just report your W-2 on your tax return as normal.
However, your employer CAN provide tax-free healthcare to your child. Assuming that the cost of "family" care is the same for employee+spouse and employee+spouse+child, you may not actually have any imputed income to deal with. And if there are separate premium bumps for the DP and the child, only the bump from the DP is taxable, not the bump from the child.
You should ask your HR or payroll department for clarification if needed.
