Of course two people with low incomes shouldn't have two kids. But (as detailed in my other comment) you need a 4 person household with 2 kids to get tax negative at $50,000/year. Even a 3 person household with 1 kid isn't tax negative in most of the US at $50,000/year.

So the other poster's argument that $50,000/year isn't so bad because of transfers, tax credits, subsidies, etc. doesn't hold up to real-world examples.