I went to that site and used their calculator for two different scenarios. Based on the same income, a married person with 2 knds would get money back. A single person with no kids would pay more.
Is there any tax system on the planet that does not penalize some, in favor of those who use more resources?
If you’re talking about the fairtax site’s calculator - it is poorly designed in that you have to FIRST go near the end to fix your assumptions - then run the numbers - for it to make sense.