The parameters to the h function are tagName, properties, children. You can skip properties if you want, and just use tagName, children. Children can either be a single node, or an array of multiple nodes.
So I think you want something like
h('div', // <div>
h('p', [ // <p>
'This text ',
h('i', 'is italic') // <i></i>
]) // </p>
); // </div>