What you are talking about is called observational gait assessment (OGA). There are a variety of OGA tools published for use by doctors and therapists with this group of patients. When an up-to-date gait laboratory isn’t available for the average parent or health care professional, then a simple video camera and OGA may be all that’s needed.
There aren’t a lot of studies to show which OGA works best and is most reliable. But since most of these tools are used to measure many children with a wide range of gait patterns, the task is much more difficult than yours. If you are the only one running the videotape and measuring your child, the results are more likely to be consistent and valid.
The main measure taken is the angle of the hip, knee, and ankle. These measures are made throughout the gait cycle. One gait cycle starts with the heel striking the ground as the person takes the first step. The cycle continues until that second heel strike of the same foot occurs.
Using stop action and slow motion of your video unit, you should be able to make these measurements before, during, and after treatment. Your physical therapist can help you identify how to make the actual joint measurements.