As a therapist, we use the therapy ball a lot with our kids. It helps them to work on balance reactions and postural control as well as provides vestibular input in a variety of directions. The kids love it because usually we sing to them and bounce them and they may even forget they are working. There are a few things to consider when using the ball though. Generally you are holding onto the child in some way so they are being provided stability (most likely at the pelvis) which allows them to only concentrate on activating their core without having to figure out how to counterbalance at the pelvis. In addition, usually kids are sitting with their feet hanging down in front of them so they are able to pull in their leg muscles to assist with activating their trunk muscles by using the overflow. If they over recruit their muscles, they can generally push against your hands to give themselves extra support. Finally (although I am sure there a quite a few more points that I didn’t bring up), if you are holding them at their pelvis or even their trunk, I can almost guarantee that you are helping them correct their posture in some way, shape or form.
Another alternative to use in conjunction or to switch it up is to utilize a platform swing. You can have the child sit in ring sitting, tailor sitting, side sitting, etc in front of you and then you can move the swing forward, backwards, sideways or even diagonal (similar to the directions you can move the ball) and see how the child reacts. This method is great because it can let the child practice sitting ‘independently’ (although with you close by) so they can practice using their pelvis to counterbalance their trunk reactions. For example, if the swing moves to the right, the child needs to stop the movement towards the right by pushing their left hip down into the surface and use their trunk muscles to shift them back to midline. In the beginning, especially with the side to side motions, kids have a hard time preparing their body for the movement and need help to not topple over. I usually start with slow, small movements until they begin to get the hang of it and then I will increase the speed or the size of the movement. By having their legs crossed in front of them they also have to work harder to isolate their trunk extensors and other trunk muscles. If you have a kiddo with higher tone, they will attempt to push their legs into extension while trying to stabilize.
Now I know not everyone has access to a platform swing so what other tools or tricks have you used to work on balance reactions on a dynamic surface, without giving the kiddo stabilization?