Hello dear I'm adding this information regarding the development of head control in cp child. I hope this will help you a little.
In abnormal process many normal components are missing. The cerebral palsied child learns to compensate and this he do by primitive movements that is to say the primitive reflexes are present in them longer than a normal child. Higher centers do not modify it. When a child develops a little more, ATNR gets stronger, so further hampers its activity of bringing things to mouth or midline orientation is not developing. Thereby visual convergence is poor and there is asymmetry in movement and development of child. These significant milestones in an abnormal child are described as blocks. From these blocks cp child develops compensation to achieve movements and thus there is abnormal motor development.
In normal development axial extension develops first that is prone lying head raising whereas proximal flexion develops later i.e. prone lying raises the head and looking around, the neck flexors also work to keep head up. In a cp child this flexion component does not develop or develops later and to lesser degree. This is because of pathology affecting the immature brain and so have hypotonic postural tone even if the extensor muscles are strong.
The lower postural tone and strong extensors muscle activity hampers the stability of proximal part of the body from which mobility is to take place.
The blocks classified in abnormal motor development are;
A- Neck Hyperextension.
Normal: In normal development head can be raised in prone, can be kept in midline and chin is tucked in and mouth is closed. After three to four month of age even in supine the child raises the head and tuck chin in. This explains that neck flexors also work symmetrically to keep the head up in midline and chin tuck in and jaw closed.
In abnormal motor development or in a cerebral palsied child the head and neck flexion does not develop or is very weak. The cp child does not bring his head to midline or don't tuck the chin in, in supine or in forearm prop position; rather in prone may be able to lift the head up and hyperextend it. He may push back strongly with his head in supine, in prone and in sitting and is so not able to sit because of increased extensor muscle activity.
Compensation: This child learns to compensate by elevating the shoulder to stabilize the head. This elevation causes hyperextension of the head and the neck and restricts the head and neck movements. Because of this, scapular movement gets blocked which hampers the movement of arm. In sitting too head and neck hyperextension leads to opening of mouth and jutting of the jaw.
Treatment of Block A. Neck hyperextension:
A. Elongating the head and neck extensors, while activating the flexors.
B. To increase the shoulder elevation or to increase depression.

Exercise 1. Supine lying:
Raise the child up from hips so that weight bearing is on head and shoulders.
Exercise 2. Side lying:
Keep under arm down and bear weight on shoulder and head on the side turned at the same time pull the upper arm also on that side of the body so that shoulder is not in elevation. In this way flexion against gravity is facilitated
Exercise 3.
In this position of body rolling the pelvis to each side facilitates righting reactions acting on head.
Exercise 4.
The next step is to roll the arm to control shoulder girdle and to rotate pelvis to get rotation within body axis.

B- Head and Neck Asymmetry:
In normal development head and neck are brought in to midline around three to fourth month of age by symmetrical action of neck flexors. This prevents the stimulation of asymmetrical tonic neck reflex thereby enhancing the midline orientation and visual convergence. Also hands are brought to midline and body awareness is developed.
Abnormal:
ATNR is strong and so child has unilateral swiping movement. Bilateral head and neck flexion is also not there. He does not have visual convergence. Since the head is constantly turned to one side the supine also rotates so a stage for Wikipedia reference-linkscoliosis can set in. The pelvis on skull side rotates forward and so femur goes into internal rotation, which later on can lead to subluxation.
Treatment:
This is same as described above plus bilateral use of arms in midline by holding toys, which will also facilitate visual convergence. Mother and child eye contact is to be developed which will indirectly develop the midline orientation, and symmetrical development of head and neck flexors, thus reducing the flexion and rotation of spine and forward rotation of pelvis of skull side thereby reducing the risk of internal rotation of femur and subluxation.