Couple of things which are striking are: your daughter's ability to do all activities and even something bizarre like continuous bar swinging under the circumstances!; and involvement of left elbow (your daughter being right handed).
Elbow is a delicate joint and surgery around this region can have some controversial results. Old unreduced elbow dislocation which is asymptomatic and hasn't fortunately resulted in any stiffness, I feel, is best left alone. Playing tennis with the right hand should not have any untoward effect on the left elbow.
Having stated all of the above, surgery at present would only aim at restoring stability and contour of the joint without any guarantee of preserving the same range of motion as it was before surgery. A future elbow replacement if required, would apparently be easier after this surgery. Articles are there supporting this.
However the final decision on remanagement would be best taken after clinical & radiological assessment of your daughters elbow.