Doubles from Ravel Morrison and Will Keane clinched a 6-3 aggregate win over Sheffield United and earned the Reds the trophy for the first time since 2003.
"It's absolutely fantastic," he told MUTV. "A lot of these boys have been with us since seven, eight and nine in the development centres.
"They've come through every age group and it's something they dream of and now they've got it.
"They're the 10th winners from Manchester United and that's a record and fantastic for them. We've had some really tremendous youth teams with top names in the history of the club so to have a little in part in it is fantastic."
United did it the hard way, having to overcome the likes of Chelsea and Liverpool on the way to glory, and the two-legged final provided a great spectacle for the supporters.
"Along the way, we've had some really tough games, some really big games and had to dig in and show a lot of the Manchester United spirit and come back," added McGuinness.
"There was no real rhythm in the first half but we got on top in the second half and I think we've deserved it - as we did in each round.
"It was a great occasion with a great turn-out for both games. There's around 20,000-25,000 here and 30,000 at Bramall Lane which are fantastic attendances so it's a credit to both teams."