One script? Well technically...
This was just a proof of concept I was thinking about for https://itch.io/jam/unity-one-script-challenge
Borne from the following thread: https://twitter.com/abhinav_Demkeys/status/1128721542018473984
Theoretically, by doing this you can have normal lifecycle hooks for all your children scripts just like you would in a normal scenario. It totally violates the spirit of the challenge, but I was curious if you could do it. You can!
You just have to do everything programatically instead of through the inspector... but maybe you'd be doing that anyway if you only have the one script.