Take a step back and think about why the build out line was put into place for 7-9 year olds.
A team has one player that can boot the heck out of the ball. So he/she takes every goal kick. Boots it up near or beyond midfield and it's race to get to the 50/50 ball. Effective, but not good soccer.
Or you have a team that doesn't have a single kid with a thunderfoot. All players from both teams camp out on the top of the 18 and wait for a flubbed kick to land at their feet and have a crack at goal. So teams leave their goalkeeper in the goal, have 1 kid line up to kick it. And leave 1 or 2 more kids inside the 18 to provide support for the flubbed kick. Now a goal kick gives the opposing team a better chance to score than if they had a corner kick. Moving the opposing team back another 15 yards means that the kicking team has a few more open options.
A goal kick is supposed to provide an advantage for the kicking team to get the ball back into play somewhat safely. In the adult game, you don't see many goals happening because of a shanked / weak goal kick. I think that in many games where you see 7+ goals scored in the younger age groups, a few of them happen because of a bad goal kick.
The purpose of the build out line is to give teams a chance to restart play and maintain possession of the ball without panicking.
Under the new rules, if a team taking a goal kick tries to boot it up the field, the build out line won't have much of an impact on either team.
I "think" the intent of the build out line is to encourage teams to play a wide/short ball to a teammate and try to "build out" from the back. Now it's up to coaches to actually use the extra space to teach their defenders to control the ball. And to teach off the ball movement from teammates to get open for a pass on the ground.
People argue that these new rules make the game "not soccer." I would argue that this makes it a bit more "like soccer" because it's making a goal kick a restart that doesn't punish a team that can't hoof it upfield. And it let's a coach move that kid with the big foot out of the back to work on other skills than kicking it as hard as they can for every goal kick.