JSX is awesome. It's just JavaScript, you can do whatever you want...

return ( <Foo> { selected ? <Bar /> : null } </Foo> );

or

let children = []; if (selected) children.push(<Bar />); if (blah) children.push(<Blah />); return ( <Foo> { children } </Foo> );

or

const bar = selected ? <Bar /> : null; return ( <Foo> { bar } </Foo> );