Guide to Find Children of Terra
They’re unfortunately very rare so your best luck is just keep looking, they’re meant to be a rebel group in hiding and not that many in numbers.
I usually found them on isolated/non connected sectors but they can also be found in connected sectors.
They are rebels, they are hiding. This means that they have fewer ships, fewer stations, their ships are not often just out patrolling, and they are unlikely to declare control of a system.
One of the best ways to find rebels in any game, world, story etc is to behave how you would expect them to behave if they had a hero complex far beyond a nor al rebel.
Soon, the dev plans to have factions able to build new stations, so if you join rebels, you may be able to help them build up a lot more.