When rescuing Halsin, whatever players do with them is players' own decision, but Halsin, on storytelling wise, who is not a shadow druid, shouldn't be the one who would willingly harm those goblin kids in a deadly way. This fight reminds me of the owlbear fight, if Halsin was in players' position fighting those two owlbears, would he decide to kill that owlbear cub? Of which I think not. And since druid are able to talk to an owlbear, while both owlbear and goblin are capable to kill humanoid beings without moral trouble to them, then what's the difference between an owlbear cub and a goblin kid?
As already mentioned, an Owlbear is a monstrosity created by magic, it's not a natural beast, it's an incredibly aggressive monster that will eat any living creature, and absolutely has the potential to upset a local ecosystem.
A Goblin is also technically, a magically created creature, they were created long ago by Hobgoblins as a serf race. Goblins are also quite aggressive, but normally don't upset the local ecosystem. It's only when Goblins come into contact with settled people that there are problems, but not always, and that's usually when adventurers get involved. In this specific instance the Goblins are being mobilised by a fanatical leadership to attack the grove. Halsin's grove. Perhaps if this were not the case he would be more lenient, but given the situation, he knows that the only way to keep his grove safe is to kill off the leadership, and as many Goblins as possible.
