And even though I change mode from pose to edit (Though I can not believe 3d animation aprication need to change pose mode for select all child bones)
The "select Similar > children " only work for one active bone.
Eg Now I hope to select all child finger bones from both hands to make bone selection groups from visible bone layer .
But blender not offer way ,without I directly select each bones, ( lasso selection etc.) or
1 I need to go edit mode
2, select r-hand
3 use select Similar >Children.
4 with shift click, change active bone as l-fhand
5 select Similar>children.
6 return pose mode
Most of standard 3d apricaiton which can pose rig, offer way to select all children bones from currentl selection.
Recently I know There are many things for key-map etc , but This standard option for posing are perfectly forgotten long time, And there seems no to do list about it. .