An easy solution to making Kagha's stance more understandable is to explain exactly what the ritual they are trying to cast does and why it can't be cast with the Tieflings still in the grove.

Have some dialogue mention that the Ritual of Thorns will create a violent growth of vines and thorns that will kill any non-druid in the grove. The ritual will protect the grove as a whole from outside threats like the goblins and other monsters that have been drawn to the grove due to the increased number of people there, but it would also kill the refugees if they stuck around.

The current story kind of implies this with the ritual being seen as a bad thing, but at least I didn't come across any dialogue that outright states it.

This would change matters considerably as the last line of defense the druids have literally cannot be enacted so long as the refugees are in the grove. Therefore the longer the refugees stay the more danger the grove is in. It isn't just about the food. The refugees are also crippling the grove's defenses just by being in the way.

You could also showcase why the refugees are a problem more too. Show how this large population of people is depleting the region of natural resources. Goodberry covers food, though every spell slot dedicated to feeding the refugees are spell slots that aren't being used to heal wild animals, mend diseased trees, and maintaining the health of the region. The tieflings would also need to be chopping down trees for firewood to keep warm at night, dirtying once clean water sources with their waste and bathing, and just all around making noise and disrupting the natural tranquility of the grove. One of the children literally tried to steal the druid's holy symbol too. I'm sure this isn't the first instance of one of the little scamps getting into something they shouldn't out of ignorance for the druids' religion and duties.

Large concentrations of humans is almost never good for the local ecosystem. More stress can be put on that point.

As far as Kagha not being very rewarding if you side with her? Yah. I think that is a major problem with choosing the villainous options in the game in general. I tried to do an evil run with my Lawful Evil Githyanki but upon meeting Minthara she didn't... Offer anything. At all. I had no clear incentive to side with her.

If Kagha suggested she could help remove the tadpole if she weren't pre-occupied with the refugee problem that would immediately make her more desirable to side with. Rather than chasing after a possibly dead Halsin your character would have a more immediately solution in front of them. In the end she can't help you of course, but have her make an effort before sending you on your way. Maybe give you some magic druid artifact for your trouble as a consolation prize.