From the IRS point of view sub companies without an FEIN are the same as the mother company. Just add their names and profit and loss on your taxes.

Sub companies with a FEIN but not incorporated are the same as the mother company, nothing extra to do here either.

Sub companies with a Fein and Incorporated as a LLC is just multiple companies, just add them to your taxes the same you would if you worked at 2 jobs.

Sub companies with a FEIN and incorporated as a Corporation is no longer a child and must do its own taxes separately.

There's nothing to "Structure" really because IRS don't care where your money is coming from, as long as you report that it is coming from somewhere so it can be taxed and tracked.

I'm not an accountant just a fellow business owner, and i've been successfully doing my own taxes now for over 20 years.