In this exercise the child lies on a firm surface with the hips and knees flexed and the feet flat on the floor.  The buttocks are then lifted until the hips are fully extended. 
      
This movement stretches the muscles that cross over the front and the side of the hips and knees, in particular the hip flexor muscles and the iliotibial band. 

If the iliotibial band is tight the child will tend to move the thighs sideways and the feet will turn outwards. 

Preventing this outwards movement of the thighs and feet when the buttocks lifted, applies an active stretch to the tight muscles.  This can be done by placing a weighted box on either side of the legs.   

Bridging for stretching the hip muscles 

Start position
Lie on a mat on a firm surface, with the hips and knees flexed and the feet flat on the floor. 

Exercise instructions
Lift the buttocks up off the mat as high as you can. 
Try to keep the thighs parallel. They should not move sideways. 

If the legs tend to move sideways, place a weighted box on either side to keep them in place.