Disposable income is Gross Income minus Taxes. A 2 parent family with 2 kids making the same as a DINK couple would have a lower tax burden and more disposable income, but the DINK couple almost certainly has more discretionary income, which money left after paying for necessities like housing, food, and medical care.

On the other hand, a 2 parent family with 2 kids is quite likely to not be making the same as a DINK couple.