All of that is reasonable and obviously missing in the whole subplot. I suggested the same because its all obvious common sense.

I would like to see clarification that stealing the idol was done to protect the kids parents - which is also really, really obvious, and not just stealing it as a prank, or to sell it. This must be available as a dialogue option in the trial.
That attempt to steal the idol can be a nice little side addition to the gang of kids that hide in the caves and one that sells "magical" trinkets. Kids trying to do something but going about it in a wrong way out of desperation and - being kids.

A group of Goonies, yeah?
And then you go "Hey kids, youre just messing things up, the situation is very tense and what you did may have just started a conflict inside the Grove and turned the Druids against you. You almost caused a catastrophe. Let me handle this."
Add some actual sliding down some tunnels Goonies style to find a small treasure and fight some monster - first hint there is Underdark somewhere bellow the Grove. There is a kid in the caves who has some kind of issue but that sub quest bugged out for me, tie all that up with a few basic options to close it.

Another important fact - issue that cannot be overlooked is the question of the attacking force. That cannot be just left out of Druids considerations. Kahga especially would be interested in who and what and why of it.
And while the ritual can maybe protect the Grove in the moment - how long that separation would last also needs to be considered - especially when a third force is in the area destroying everything and unbalancing the nature.
For christ sake the inner Druid Sanctum is full of frescos depicting how Druids fought the dark forces, right?
Harpers are involved, in the area.

Would the Ritual be completely impervious to everything outside forces can do? Do they mean to stay locked up forever? Is that really a solution?

What if the Grove can be breached through the Underdark - connection you discover in the Gonnies subplot?

Could evil players use that later on to breach the sealed Grove from the inside as it were? Trade that info or abuse it themselves? Run into a cabal of Shadow druids that took over the Grove once it was sealed and are trying to corrupt and expand the ritual so it destroys everything?

All these options become naturally available if you just respect the established specifics of the lore and the setting and basic common sense.

I dont think Kahga needs to offer another tadpole "solution" or be told about it. We are already telling about our tadpole to way too many people as it is. We dont need another false hope or giving that info to people we dont know. (Unless its done as a obvious mistake with bad consequences later on)
The evil players do not need to become buddies with every other evil character or plot. In fact, being evil would work against that.
Kahga, being a Shadow Druid would consider everyone else the enemy and the player would discover that and her situation then needs to be solved along those lines. Even if the player is playing evil Kahga and Shadow druids would be competition, not allies.

This would also remove the "im evil lets just kill everyone muahahaha" evil gameplay a lot of players are already complaining about, and give them some more interesting options, gameplay and consequences.