As mentioned above, your best bet will be using flash (or flood lights) and tripod.
But you do have the Canon powershot Pro1 with a "L" lens boasting f/2.4-3.5 with zoom range 28-200 mm. So you may want to try a few tricks and see whether it will work okay with you in low light without flash. This may work if your grandson just moves a little and most likely not work if he's jumping up and down or in very low light situation:
1. Try the manual mode
2. Set the aperture to 2.4 (lowest number, which actually gives you the widest opening to allow more light into the sensor)
3. Set the shutter speed to 1/250. If it is too dark, go down to 1/125 (slower shutter speed, will look brighter but may gives mild blurring). Faster shutter speed 1/500 usually is not possible in low light, but you can try it if you still have blurring at 1/250.
4. If your picture at 1/125 is still too dark, go up on the ISO. Your camera's ISO range is 50-400. The higher ISO number will help to make your picture brighter, but will introduce noise and make the picture grainy. Start with ISO 100, and go up as needed to make the picture just bright enough for your taste. You can of course slow down you shutter speed further to make the picture brighter, but may have more blurring as you do so.
5. Set you autofocus (AF) to continuous, so that focusing will be automatically adjusted as your subject moves.
6. Try not to zoom in too much (you can zoom in a little since you have wide angle), instead get closer to your subject.
7. Mount your camera on a tripod if possible, but this may not be possible when you're alone with a 2 year old. Then hold your camera with 2 hands, press the button half way to focus on your grandchild's face or eye, and gently press the button to take the picture while you exhale (holding breath and inhale may increase body shake). If your grandchild's face/eyes are in focus, then his hand or body slight blurring may actually add a sense of motion to your picture. If you focus on his body and get a blurry face, then the picture will look horrible. When you set the aperture to 2.4 wide open, your depth of field will be shallow, stay focus on the face and eyes.
8. Try to light up the room as much as possible, or find the brightest spot indoor to do your action photo.