The group has also promoted flag football, which has no tackling, with success. Participation in the game, which is typically played by younger athletes, grew 8.7 percent last year, Mr. Hallenbeck said.

Even so, participation in tackle football by boys ages 6 to 12 has fallen by nearly 20 percent since 2009, though it rose 1.2 percent, to 1.23 million, in 2015, according to the Sports and Fitness Industry Association. Schools in several states — including Maine, Missouri and New Jersey — have shut their tackle football programs because of safety concerns and a shortage of players.

The participation declines in tackle football are worrisome not just to youth football organizations like Pop Warner, but to the N.F.L., which sees youth football as a way to develop future fans and pro players. The N.F.L. has given U.S.A. Football tens of millions of dollars to promote the youth game, and the league’s presence was felt at the convention, which was held just a few miles from where the Pro Bowl was played.

U.S.A. Football began exploring new ways to play the game in 2015. Unlike sports like baseball — which has a progression of levels, from T-ball up, suited to each age group — football had few alternatives to the flag and tackle versions. The new format, called modified tackle, is a way to give nervous parents an alternative. Coaches would also rotate players in different positions during games to give everyone a chance to carry the ball and avoid mismatches between large and small kids.

The first modified tackle scrimmages were held in September with youth teams in Cleveland. Chuck Kyle, the football coach at St. Ignatius High School, who ran the scrimmages, said that though much more work was needed to determine if this version of the game was safer, the initial evidence was positive.

“By bringing the field in, first of all, I think there’s better form tackling because less speed, less momentum, more one-on-one tackling,” Mr. Kyle said. “I didn’t see as many pileups, because there’s seven people” on a side, not 11.