ui_panels.py

import bpy
from bpy.types import Panel

class RENDER_PT_Panel(Panel):
    bl_space_type = 'PROPERTIES'
    bl_region_type = 'WINDOW'
    bl_context = "render"
    bl_label = "Parent Panel"
    
    @classmethod
    def poll(cls, context):
        return True
    
    def draw(self, context):
        layout = self.layout

class RENDER_PT_Panel_Child_A(Panel):
    bl_space_type = 'PROPERTIES'
    bl_region_type = 'WINDOW'
    bl_context = "render"
    bl_label = "Child_A"
    bl_parent_id = "RENDER_PT_Panel"
    
    @classmethod
    def poll(cls, context):
        return True
    
    def draw(self, context):
        layout = self.layout

class RENDER_PT_Panel_Child_B(Panel):
    bl_space_type = 'PROPERTIES'
    bl_region_type = 'WINDOW'
    bl_context = "render"
    bl_label = "Child_B"
    bl_parent_id = "RENDER_PT_Panel"
    
    @classmethod
    def poll(cls, context):
        return True
    
    def draw(self, context):
        layout = self.layout