Her comments follow growing controversy over President Donald Trump's "zero-tolerance" immigration policy.
Earlier Melania Trump made a rare statement expressing concern.
Mrs Trump "hates to see children separated from their families", her spokeswoman said.
She repeated her husband's call for "both sides" to work on immigration reform as a solution. However, fact-checkers point out that the policy was introduced by Mr Trump's attorney general and does not require congressional action to be stopped.
In a recent six-week period there were nearly 2,000 family separations following a crackdown on illegal border crossings.
Adults who try to cross the border outside of official entry points - many planning to seek asylum - are placed in custody and face criminal prosecution for illegal entry.
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What's been said?
Laura Bush, wife of the former Republican President George W Bush, launched an outspoken attack on the policy."This zero-tolerance policy is cruel. It is immoral. And it breaks my heart," she wrote in the Washington Post.
"Our government should not be in the business of warehousing children in converted box stores or making plans to place them in tent cities in the desert."
Earlier Mrs Trump said she "hates to see children separated from their families and hopes both sides of the aisle [Republicans and Democrats in Congress] can finally come together to achieve successful immigration reform".
"We need to be a country that follows all laws, but also a country that governs with heart," her statement added.
Who is to blame?
Mr Trump has said a law "Democrats gave us" is responsible for the policy, but it is unclear which law he is referring to.In a tweet on Saturday he urged Democrats to work with Republicans to create new legislation.
