select_children.py

bl_info = {
    "name": " Select children",
    "author": "icyp",
    "version": (0, 1),
    "blender": (2, 69, 0),
    "location": "View3D",
    "description": "select Children",
    "warning": "",
    "wiki_url": "",
    "tracker_url": "",
    "category": "3D VIEW"
    }
    
import bpy
from bpy.types import Operator
      

def select_children_recursion(obj):
    if obj.children :
        for child in obj.children:
            child.select = True
            select_children_recursion(child)

def select_children(self, context):
    for obj in context.selected_objects:
        select_children_recursion(obj)
            
class OBJECT_OT_select_children(Operator):
    bl_idname = "obj.select_child"
    bl_label = "select Children"
    bl_options = {'REGISTER', 'UNDO'}

    def execute(self, context):

        select_children(self, context)

        return {'FINISHED'}
    
class select_children_button(bpy.types.Panel):
    bl_label = 'Select children'
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'TOOLS'
    bl_context = ''

    def draw(self, context):
        col = self.layout.column(align=True)
        col.operator(OBJECT_OT_select_children.bl_idname)


# Registration

def register():
    bpy.utils.register_class(OBJECT_OT_select_children)
    bpy.utils.register_class(select_children_button)

def unregister():
    bpy.utils.unregister_class(OBJECT_OT_select_children)
    bpy.utils.unregister_class(select_children_button)


if __name__ == "__main__":
    register()