try this way:

class Materials(models.Model):

_name = 'materials.materials'

task_id = fields.Many2one('project.task')

is_child_material = fields.Boolean("Is Child Material", default=False)

class Task(models.Model):

_inherit = 'project.task'

materials_ids = fields.One2many('materials.materials', 'task_id')

parent_materials_ids = fields.One2many('materials.materials', 'task_id', domain=[('is_child_material', '=', False)])

@api.onchange('materials_ids')

def _onchange_materials_ids(self):

# Set the 'is_child_material' field to True for materials added to subtasks

for subtask in self:

subtask.materials_ids.filtered(lambda m: not m.is_child_material).update({'is_child_material': True})

you want to prevent materials added to a subtask (child task) from being displayed on the parent task. In Odoo, by default, related fields like One2many fields will display all related records, including those created on child records. To achieve the behavior you want, where materials added to a child task are not displayed on the parent task, you can use a computed field to filter the related materials based on some criteria.
from odoo import api, fields, models