At a public health summit in Washington, she hit back after the Trump administration loosened nutritional standards aimed at making US school lunches healthier.
"Think about why someone is OK with your kids eating crap," she said.
One in five American children is obese, government figures show.
In thinly-veiled criticism of the policies of the new administration, Mrs Obama told the audience: "This is where you really have to look at motives, you know.
"You have to stop and think, why don't you want our kids to have good food at school? What is wrong with you? And why is that a partisan issue? Why would that be political? What is going on?"
She added: "Take me out of the equation - like me or don't like me. But think about why someone is OK with your kids eating crap.
"Why would you celebrate that? Why would you sit idly and be okay with that? Because here's the secret: If someone is doing that, they don't care about your kid."
Michelle Obama encouraged healthy eating to combat obesity in children
While in the White House, Mrs Obama championed the "Let's Move" campaign, which encourages exercise and healthy eating among young people.
The 2010 Healthy, Hunger-Free Kids Act made federal grants for school meals conditional on reductions in calories, sodium and trans fat content and increases in fruit, vegetables and whole grains.
However, earlier this month, US Agriculture Secretary Sonny Perdue postponed reductions in sodium, relaxed requirements for whole grains and allowed sweetened flavoured milk back.
He said some aspects of the standards had "gone too far".
The agriculture department said the change would give schools "greater flexibility".