Casuals won the play-off final last Saturday with Kelly scoring one of the penalties in the climactic shoot-out against Corinthian-Casuals, with both players scoring in the 5-2 semi-final win at Cray Wanderers.
Over the season-long loan spell at Walton, striker Kelly (pictured scoring for Walton) scored 40 goals in fifty appearances, whilst winger McCormick hit the target 9 times in his fifty-two games.
Both 19-year-olds are expected to play a role in the Magpies` Vanarama National League squad next season.
Joining them will be creative midfielder James Comley, who has committed himself to United for a fourth season.
This leaves just a handful of the current squad still to make a decision about their future, with Sean Marks the only confirmed departure.