For tax purposes, your family is considered to be everyone you put on your income tax return. This applies even if you do not claim a dependent that you are eligible to claim on your return.
If you qualify as head of household, then your tax family is your seven-year old, and you.
I have not read over the 300% to 400% FPL very well. Therefore, I cannot comment on that. However, try calculating your potential tax penalty, if any, by taking your penalty to be either:
1) ($142.5) $95 for yourself + $47.50 for your child
2) (Your total household income minus your filing threshold) * 1%
Your penalty should be the greater of those two.
Also, try investigating if you qualify for an exemption.
I hope this helps.