Pieter Meyns has made a gait markerset available for children with cerebral palsy (hemiplegia n=5, 9.00 2.28; diplegia n=4, age 10.50 1.66) and typically developing children (n=5, age 8.40 1.50). You can access the data at https://simtk.org/projects/cp-child-gait/. SimTK has a number of other datasets available, which you might search to see if there are other ones that meet your needs.