The constructor with _disableInitializers() runs on the implementation contract, whereas the initialize() function runs in the context of the proxy. See Is _disableInitializers necessary? - #2 by ericglau for reasoning.
In any case, I've included
_disableInitializers();in my parentconstructor, should I also include it in the childconstructoras well
If it's already in the parent, it's not necessary to include it in the child.