What are some ways I can teach my kids 12-14 year olds to defend a dominant big man? We are having trouble with positioning and we are giving up too many easy baskets underneath.

You can try dead fronting the post player and giving help IF he gets the ball... double team him with the nearest player and everybody rotate to the basket.... force him to pass the ball.
Make sure that you put a lot of pressure on the player with the ball... don't give him an easy look for a lob pass... at that age... it might be a tough pass anyway.

Ken