Yes, you will be considered as a household of two when you apply for premium tax credits. However, your grandchild will be considered as her own household for Medicaid and CHIP and your income will not count in determining her eligibility for these programs. Assuming she does not have her own income she will likely be eligible for Medicaid or CHIP and not eligible for premium tax credits for coverage in the Marketplace. You could of course purchase coverage for her in the Marketplace but you would not be eligible for a premium tax credit to help pay for her plan. Whether you could include her on your policy would depend on what insurers offer in your Marketplace.