Are you looking for a change over time or just a comparison to the norm reference? The Peabody is a good discriminative test and is used a lot in physical therapy to determine if a child is developing atypically it is not great at showing whether treatment or an intervention is effective.
I would like to be able to derive norm-referenced scores and also to use it for an outcome measure for motor development in an RCT, so this sounds ideal. I had looked and there are few items in scome sub-scales with a good deal of overlap so I was a little concerned about poor discrimination across ages, but all the children will (hopefully) be assessed around the same age. Thank you for your response.