For some components, it makes no sense to have more than one child. For these cases, you would use the React.Element type instead of the React.ChildrenArray type:

// @flow
import * as React from 'react';
import Child from './Child';

type Props = {
  children: React.Element<Child>,
};
 
const ParentWithOneChild = ({ children }: Props) => (
  <section>
    <h2>Parent With One Child</h2>
    {children}
  </section>
);
 
export default ParentWithOneChild; 

As with the example before this one, you can still specify the type of child component that is allowed. In this case, the child component is called Child, imported from './Child'. Here's how you would pass this component a child component:

<ParentWithOneChild> 
  <Child /> 
</ParentWithOneChild> 

If you were to pass it multiple Child components, Flow would complain:

    Property `children` is incompatible:
         24:         <ParentWithOneChild>
                     ^^^^^^^^^^^^^^^^^^^^ React children array. Inexact type is incompatible with exact type
          6:   children: React.Element<Child>,
                         ^^^^^^^^^^^^^^^^^^^^ object type. See: src/ParentWithOneChild.js:6
  

Once again, the Flow error message shows you exactly what is wrong with your code and where.