I released it, although as others have noted, the main motivation was probably to antagonise the Crones. More than anything I did not trust them, and did not want to do anything to potentially strengthen their hold over the swamp. I felt that letting the tree spirit go, knowing it would save some children - at least in the short term - was worth whatever trouble it might create, especially if it weakened the Crones' power as well.
The consequences of that decision? Yeah, hit me in the feels. But I don't regret it.