To the Other Side (2004)

Two little boys aged about 8 or 9 strip down to their underwear for a swim in the lake. After splashing around in the water for a bit, both boys decide to take off their underwear and whip them at each other. They do this while still in the water, with only their torsos exposed. When they get out of the lake, both boys are now naked; brief frontal nudity for a couple of seconds.