I know you're in the USA Katy, where social welfare is very different to here. In Aus, child support and family tax benefit is generally NOT means tested. It is income-based. So yeah, theoretically someone with $5-8m in assets could still get it.
But i think the important point here is that it was *only* 15k a year. It is not a lot of money when you consider the kids school fees were 40k+ a year. Simon was paying those prior. Her math is illogical. I think she threatened the child support and family tax benefit to make him comply, rather than having done the math - because, she was an accountant, she surely knew she wouldn't be eligible for much from Simon, but I think she tried to threaten him which didn't work.. he seemed disengaged and called her bluff (perhaps, finally?) "Go right ahead Erin, you will be worse off if you take that tactic with me..." (is what i imagined him projecting to her).