I think the grandpa's decision was good since it meant that the kids were heard and validated in their thinking. He didn't deny or condemn their wants which encourages them taking the initiative and reassures them that they can trust/confide in talking to an adult close to them. If the grandpa denied them, then it'd make them close off from confiding to an adult figure. It may not be a big deal but the act of rejecting a child's initiative is a surefire way of ruining their confidence.
Children are very sensitive to the people around them and what may be casual to an adult, it can be life changing to them. It's the same issue that Hayato had earlier in the story about his offhanded comment with the big meals the family had. And how his mom pulled him aside and apologized. You don't know how your actions will affect the child if you condemn them bc you thought you were doing the right thing by excluding them from their own autonomy.
I'm not saying Kojiro is doing the right thing in the big picture, but for Gakuto and Waka, he did right by them. He even gave them opportunities to step down and when they didn't, he respected their decision. Children are people, not something you can make choices for and hope for the best. If they wanna do something with that much emotional resolve, the best thing to do is encourage them and then offer an alternative if they need one.
I'm sorry this is such a big post shdhdsd i get very involved when it comes to emotional development and i just love this family dynamic so much!! Thank you for the update!