It's hard to teach exercises for infants cos it's hard for them to do exactly what u need them to do. You can educate the patrents to hold the baby on the problematic side and let the righting reaction of the head stretching the SCM by side flexion.

I've done a post-op rehab to a 14-year-old. In the first few days his neck is immobilized in a soft collar. The following physiotherapy sessions are mainly passve stretching the SCM(exactly what to do to the infants, which is quite tiring to the therapists) Ultrasound and postural correction of the neck. The progression was good without any traction so I don't know if traction can be helpful.

The most important part is the postural correction. Because the child have already used to be in the habitual posture and it needs time to adapt and rehabilitate what is the true correct posture of the neck after the SCM is surgically released.

Hope these informations are useful