In a ball crawl like this one, have your child sit down.

Pick up the corners of the ball crawl quickly back and forth, to make the balls inside like popping popcorn.  Try to get the balls to jump over your kid’s head!
Another fun variation on this is the “ball drop”.  Have your child fill a plastic drawer or bag with the balls from the ball crawl.  Then, they climb inside and you dump the balls on their head in a “ball drop”.