dear Fyzzion,
sorry I have to refine this a little.
The GMFM is validated for CP that is true but on the website you can read that; "While the original measure was designed and
validated for children with cerebral palsy, there is evidence that the GMFM-88 version of the measure is also valid for use with
children with Down Syndrome. The updated GMFM-66 version is ONLY valid for use with children with cerebral palsy.
As the GMFM-88 samples motor skills that are typical of normal developmental milestones, it may be useful for children other
than those with whom it has been validated; however reliability and validity should be established prior to using it with other
groups of children. "
So it is a good reference for the milestones.... but you do need to buy the book (including DVD).