Since 1981, every major tax cut in the U.S. has included tax relief targeted at parents. The 1997 budget deal coupled a capital-gains tax cut with the creation of a tax credit for children. President George W. Bush expanded that credit as part of his 2001 tax cuts, and accelerated the phase-in of the expansion in his 2003 tax cuts.
The policy rationale for such relief is that parents are overtaxed: They pay too large a share of the burden of modern government, which depends on their financial sacrifices in raising children, but does not adequately account for the contribution they make. (See here for more.) The political rationale is simpler: Pro-family tax relief is popular, in part because it directly benefits a lot more middle-class people than other tax-cut ideas.