Tickle your child (in the area of the abdomen). You want to see a powerful contraction of the muscles of the trunk, bringing elbows and knees together.
Try stretching and bending again, moving the child's spine in two directions.
You can sit either at the head or pelvis of the child.
With a heavy child, you can put some support under the pelvis, this makes it easier.
Hold the knees and start rolling the pelvis toward the face.
Pick the easiest direction. This is the most important part! Delicately feeling for the vertebrae that are already able to perform the movement is MUCH more important than lifting high.
By lifting the pelvis to different heights you target different vertebrae. "Higher" is not "better".
Watch the breathing. If it stops or the child feels uncomfortable, do less.
Do small circles with the pelvis at different heights.
Always go in the direction the child is already turning the head. Some children can easily bring their knees to the ears.