Robert Sanfilippo, 45, who manages the Long Island Vengeance team of 10- to 11-year-old boys, got into a dispute with another coach during a tournament in May, police said.
He bought an untraceable phone, police said, and in August, he started sending text messages and photos, threatening to harm the other coach, his son and his wife.
Commack resident John Reardon said he, his 10-year-old son and his wife were the ones being stalked for about two weeks.
"I didn't know it was him at first," Reardon said. "It was nerve-wracking. I couldn't sleep. When he suddenly started mentioning my son by name, it just hit me that it was this guy."
The texted threats included blurry photos that appeared to be taken from a long-distance zoom lens, with one showing him waiting with his son for the school bus and another of his wife taking packages out of the car, Reardon said.
Text messages made fun of his wife, him and house, Reardon said, but "when they started saying he'll pick up my son at the bus stop, 'Tell him not to talk to strangers,' that's when it got crazy for me. That's when it got scary and that's when I went to the police."
Sanfilippo of 20 Timber Ridge Dr., was held pending Saturday arraignment on 20 counts of aggravated harassment and one count of stalking. He was arrested by Fourth Precinct crime section officers.