- Gundam 00: Gundam as a terrorist - Iron Blooded Orphans: Gundam as a (child) mercenary
After this I you want to watch the original universe (Universal Century) you can start with the 3 movies that summarize the OG series. They are legally on YouTube (search for gundaminfo)
I know what you're saying, but except for Hathaway, child mercenaries are in every Gundam installment?
I love those alternate universes, but those are so different to other gundams (UC and Seed?) both are very "what ever is cool now, but with gundams"
But they are from the 70s, so don't expect top notch animation and scripts (it's a product of its time).
After that you can go Char's Counterattack and Gundam The Origin (this is a recent series, so more polished animation). Later maybe try watching Zeta.
As another comment mentioned you also some shorter UC side-stories like War in the Pocket, The 08th MS Team and Stardust Memory.