Children develop motor skills (e.g. sitting, walking, running), at different rates and it is normal to see a variation between children in the age at which they develop these skills.
A loss of previously-attained motor skills (motor regression) or a reduction in motor skills may be caused by a serious illness, including a brain tumour.
Brain tumours can initially cause subtle changes in motor skills such as a change in hand or foot preference or loss of learned skills, e.g. ability at computer games and handwriting.