Goals in each half from Ben Reeves and Jason Banton saw the Dons establish a comfortable advantage which was only threatened late on after Roy O'Donovan reduced the arrears.
Dons would have been out of sight had it not been for Northampton goalkeeper Matt Duke, whose heroics helped Bradford into last year's final. Duke kept Cobblers in the contest with a string of fine saves in either half.
But it was the Dons youngsters who caught the eye, particularly Dele Alli, who Robinson rates very highly and is determined to hold on to, at least for the time being.
"The average age of this side was only 20 but we want to give them the chance and they looked very exciting," Robinson said.
"Northampton was always going to be a tough test for them but they conducted themselves very well.
"Dele is not for sale, it will take millions to take him but I don't want to sell him and nor does the chairman. There will a day when we will have to because he's destined to be a great player but great players have a path to take and I believe that's with us because he's playing in the first team.
"But we've got several very good young players, we coach them right and we play them."