What worked for me and my son's 2nd grade team was to do focus on boxing out instead of going after the rebound.
I took two kids at a time, placed a ball on the floor at center court (so there was circle perimeter going around where the ball was placed), then when I blew my whistle one kid had to try to get the ball and the other kid had to box him out and keep him out of the circle and away from the ball for as long as he could.
During games we emphasized turning around, boxing out and looking for the ball as soon as a shot goes up. Once they got the boxing out concept, it made rebounding a lot easier.
good luck.