Flutter children node checkbox not selectable

I have a treenode implementation i am doing right now ut run into a blocker. When i create parent node and children nodes, i am able to select the parent node checkbox but unable to select the children node checkbox. I want to be able to select the children node checkbox as well so that i can be able to retrieve data from the parent Treenode and store them in supabase. Howver, i don't know what i am doing wrong in the code.

This is the simplified version of the feature code: https://www.dartpad.dev/?id=d644a706f84cb39588a763a767a2e15c

I tried this today again but no success. I have google everything possible, search Stack Overflow yet no solution. I expected to be able to select children node. That is if a parent node have multiple children nodes, i can be able to select a single child node but at the moment i have to select the parent node which in turn select all it children