Not sure how old child is but if they have had a head tilt for a long time (congenital muscular torticollis?) remember that they have accommodated to that position for so long that tilted feels like horizontal - their visual, proprioceptive and vestibular systems have to be retrained.  Try using a mirror with verbal cues to correct alignment, prone activities or wheelbarrow position looking in a mirror will elongate the SCM, then add forward reach with either UE.  Aquatic therapy is great too because if they tilt their head in the water, their body will not remain well balanced for floating in prone or supine.  (Use flotation supports if needed like pool noodles).