Title holder Bayern Munich kicked off the 2020-2021 campaign on Saturday (AEST) with a record 8-0 thrashing of Schalke, but Dortmund's youngsters also dazzled at home to Gladbach.
"It was a really good start to the season. We have a lot of young guys, it's fun to play with guys near my age," United States Under-17 star Reyna said.
"This is an exciting group and we have to keep working."
Haaland converted a second-half penalty, then again showcased his clinical eye for goal by finishing a superb counter as Dortmund attacked from its own goal.
Coach Lucien Favre banked on youth by starting two 17-year-olds in Reyna and England Under-21 midfielder Jude Bellingham, who made his league debut, with Sancho and Haaland, both 20, leading Dortmund's attack.
Reyna, making just his third league start, paid back the faith by scoring the opening goal and winning the penalty Haaland converted.
Reyna drilled home Bellingham's superb pass from a tight ankle on 34 minutes, then had his ankle clipped after a darting run into the area to win the penalty for Haaland.
Sancho tried to convince Haaland to let him take the spot-kick. "I was confident whoever took it, would score, which is the most important thing" Reyna said.
Haaland then stole the show with a stunning end-to-end sprint to support Sancho's break before crashing home Dortmund's third goal on 78 minutes.
The towering Norwegian, the first player in Bundesliga history to score a hat-trick from the bench, now has 18 goals in 20 games for Dortmund since his arrival in January.
For the first time since March, some German league clubs were able to welcome back limited numbers of fans, depending on permission from local health authorities.
The biggest crowd of the day was in Dortmund, as 9300 cheered the impressive home win.
"It's amazing to have fans back, they support us so much," Reyna said.
Having finished runner-up to Bayern for the past two seasons, Dortmund is on a collision course with the European champion, which it will meet in the German Super Cup showdown on 1 October (AEST) in Munich.