Please see my comment to the question. Windows in WPF are called windows not forms, the are derived from the class
Those windows are designed to be top-level Window windows, not children of anything except a desktop. Even though you can use a dirty raw Windows API trick to make one a child of another, you should never do it, this would be a great UI abuse. There are numerous legitimate ways to organize a set of nice hierarchical container UI elements, which would be different kinds of panels, tab controls, and the like. This is matter of regular design work to decide which UI elements to use and how to organize the layout and their collaboration.
To start with, you need to learn WPF Layout Model