Yes, you are approaching the tax family size incorrectly. If you look at Form 8962 instructions for Line 1, tax family size, "your tax family generally includes you, your spouse if you are filing a joint return, and your dependents." It is not based on how many people are receiving marketplace insurance. The number is only modified if you are allocating a plan and the taxpayer receiving the allocation is receiving 0% of the plan. In that case, the family size is 0.
Even though only the child your partner is claiming is being covered by marketplace insurance, her family size would be two. The calculation is based on the poverty level for the household size, not the amount of people receiving coverage.
If TurboTax was calculating her premium tax credit based on a household size of two and you were using a household size of one, then that would result in a larger credit using TurboTax's calculations.
On your form it might not matter because if your income is 400% above the poverty level based on your household size, whether your household size is two or three, then it would not impact the calculation. To put it into numbers, the poverty level for 2 in most states is $17,240 and $21,270 for 3. If your income is more than $85,080 you are not receiving premium tax credit regardless of whether your household contains two or three people.
