Well, if you are a simple family where everyone is biologically related and living together, then things are pretty simple in the end. The issues come up with mixed families, divorced parents, etc.

As for the dollar values, if you make $30,000 and have 2 kids, you can usually get a $6,000 tax credit or more. The U.S.'s support for working low-income families is carried out through the tax system. Put another way, tax credits are one of the U.S.'s most important social safety nets.