2D legacy animation. change child sprite renderer won't work

as the title says, sprite changing won't work with child object. I have a player object with some parts that changes (face and arms) but when I try to change the sprite at certain frames, it won't work.
eg.
Face base sprite : face_0
frame 0 : face_1
frame 20 : face_2
frame 30: face_1

when I play, the face sprite is face_0 (base) and won't change at all. is this a bug ? this works fine with root game objects.