Would William's child become monarch over harry?

Yes. The line of descent from Elizabeth II is to her eldest son, Charles ,then to his eldest son, William and to his eldest son. If William has daughters only then the crown would pass to his eldest daughter. If he has a daughter and then a son, under present UK law the crown would pass to his son.

The crown would only pass to Harry if William died childless or abdicated. If Charles abdicates the crown would pass to his brother Andrew, Duke of York and William would not inherit. If Queen Elizabeth II were to abdicate the crown would pass to Viscount Linley, eldest son of her late sister, Princess Margaret.