cdaref wroteFull speed or not, the OP's situation was that he "slams" into the defender with LGP. I guess I agree with Jimgolf.
I've seen kids drive the lane and slam into the defender, and unless he's coming from half court building speed, this "slam" isn't normally that hard. Usually, it looks a lot worse for A1 because of the complete and sudden stop.
I'll reiterate. If the kid comes in like a cannon ball, call the foul. If the defender gets hit that hard, though, there's going to be some displacement.