The "tan faced children" that Whitman is referring to covers several different possibilities.  Whitman was a great champion of the working man, the laborer, the farmer out in his field, the sailor out at sea-those people who went out every day and sweated hard for their daily bread.  So, the tan-faced children could be referring to that entire working class of people who, through exposure to the harsh elements, are browned, wrinkled, leathered, and toughened.  Much of the poem does address hard-working people who are laying the foundation for this great country.

It could also apply directly to the title, which is repeated throughout the entire poem, to the actual pioneers who settled, cultivated, and civilized the country as a whole.  Again, exposure from working hard on farmland, or crossing the plains to settle the west, made these people tan-faced and tough.

Using children as a descriptor refers to the large number of immigrant children who ended being in these positions.  Also, it is a way for Whitman to express affinity with them, to feel like he has a connection to them.  He isn't passively describing them and their labor, he is one with them, related, there with them the entire way.  Whitman loved to feel a part of all people, and often described himself as related to humankind through nature and their work.

Those are a couple possible interpretations, and I hope that it helps!