Examining second 12 at ball contact
, her arm and racket are in a straight line and her wrist, in my opinion
, looks to be at a stressful angle. ?
For the current pro serve, where internal shoulder rotation(ISR)
is the largest contributor to racket head speed at impact, there should be an angle between the forearm and racket at ball impact. This motion is not obvious or intuitive so self-taught players usually have no idea.
See high speed video of serve below and other serves where the thumbnails clearly show the forearm-racket angle and the wrist angle for pros.
Compare to your daughter's forearm-racket angle, compare wrists.
There are several recent threads discussing this point. The ISR component of the motion is usually misnamed 'pronation' in tennis usage.
See replies #18 & 20. Especially view the Ellenbecker video in reply #20 on shoulder anatomy & at minute 8, shoulder positioning at impact to minimize injury risk.
I'm not an instructor and have no idea what a child that young should be doing.