Foster care advocates say the government won't likely be able to reunite thousands of children separated from parents who crossed the border illegally, and some will end up in an American foster care system that is stacked against Latinos and other minorities.

With few Spanish-speaking caseworkers, it's a challenge tracking down family members of the children who live south of the U.S.-Mexico border, and other relatives living in the states might be afraid to step forward to claim them because of fears of being detained or deported themselves.

Many complications have arisen for these separated...