Is this possible with GLFW? AFAIK you can't use GLFW to create a child window per se, so the question probably boils down on how to turn a GLFW created, opengl enabled window into a child window and assign the parent window.
The short answer is to use a UI extension like DearIMGUI, wxWidgets, or QT.
I have pondered the question too, so much so that I'm partaking in the following. So instead, using conversion types from GLUT (instead of using a wrapper). I am plugging my extra time outside of development in to making a GLFW dependent extension, in the form of a
.h file, which allows you to create UI's, text input/output (though GLFW already supports this part (if you appropriately read documents)), image texture loader, and a separated loader for 3d models/animations. Trying to assist in making development easier for GLFW users. 1 plugin to substitute all the additional plugins.