Jacob Brown's penalty gave Barnsley the lead, but by the 14th minute Leicester were 3-1 up thanks to Darnell Johnson and a double from Sam Hughes.
Layton Ndukwu hit the fourth before the break and an own goal from Louis Wardle and strikes from Josh Gordon and Hamza Choudhury completed the scoring.
Leicester top the group with 10 points, although Everton on three points, and level with third-placed Barnsley, have two matches in hand.
Portsmouth are bottom with one point.
See: Leicester report
Leicester: 3 Jan v Spurs (H) PL2
Barnsley: 3 Jan v Everton (A) PL Cup