Why did Trump say that the coronavirus can't be transmitted from kids to adults?

Source? How long ago? I'm guessing you don't know?

Because he’s a racist moron 

He didn’t say it can’t he said that it is not a major mode of transmission and he is correct.

Scientists at Institut Pasteur studied 1,340 people in Crepy-en-Valois, a town northeast of Paris that suffered an outbreak in February and March, including 510 students from six primary schools. They found three probable cases among kids that didn’t lead to more infections among other pupils or teachers.

The study confirms that children appear to show fewer telltale symptoms than adults and be less contagious, 

Keep in mind fetuses are smarter than him.

Trump said it's less common to transmit through kids, retard. Fake news demonrat *smh*

He's the Scientist General.

Perhaps you misunderstood, Trump misspoke, or it was edited that way (like CNN does to help them vilify anyone they hate). 

Did he say that, if so he is a bit incorrect because it can happen but it is rare and not a common mode of transmission.

There is some evidence that the viral load in children is too low to cause easy transmision to others, but that does not mean there is no risk.