Flagrant Fouls are usually savage and violent, intent to injure type of contact. That is totally judgment on your part. You were there and if all you have is a push in the back, I probably would go with only a Intentional Foul. But if you think he was clearly trying to hurt the shooter, then go with the Flagrant. Sometime the level of the players makes a huge difference in that. Not sure a 5th grader is really trying to hurt someone. But then again, I was not there and did not see the play. When in doubt, go Intentional.
Peace