The Bluebirds will face Swansea City in the Welsh Youth Cup semi-finals.
Cardiff City scored 10 goals against Llanelli in a one-sided last eight tie, while the Swans defeated Merthyr Town 5-1 at Landore.
In the second semi-final The New Saints will play Cardiff Met or Llandudno, whose scheduled match was postponed because of bad weather.
Both semis will be played on Sunday, March 10.
Dan Griffiths (4), Sion Spence (2), Dan Martin, Laurence Wootton, Niazana Mayembe and Kieron Evans were the scorers for Cardiff, who led by four goals at half-time.
Wotton also hit a post, while Llanelli had Jamie Owen sent off after 68 minutes for a second bookable offence.
Cardiff City: Jordan Duffey, Dan Martin, Ben Margetson, Jac Davies, Joel Bagan, Sam Bowen (capt), Harry Pinchard (Rubin Colwill 57), Niazana Mayembe (Kieron Evans 54), Laurence Wootton, Sion Spence, Dan Griffiths. Subs not used: George Ratcliffe (gk), Connor Davies, Trystan Jones.
Llanelli Town: Rees, Thomas, Howard, Davies, Saer, Owen, Evans, Wand, Cutler (Howells 50), Cooke, Jenkins. Subs not used: Millard, Kivi, Wright, Howells, Nicholas.
Swansea City booked their place in the semi-finals with a 5-1 win against Merthyr Town at Landore.
Young Swans, looking to win the trophy for a 10th year in succession, found the net through Owen Evans, Mason Jones-Thomas, Jayden Reid, Bradley Gibbings and an own goal, despite losing Dan Williams to a red card in the second half.
The final score of 5-1 was harsh on Merthyr Town, who played far better than that suggests.
Cavan Williams scored their goal and hit the bar from a free-kick.
The New Saints won 5-1 against Cefn Druids, who had their goalkeeper sent off inside the opening 10 minutes. James Parry and Ethan Short scored to put TNS two goals ahead at half-time, while Parry, Owen Jones and Tai Jones completed the scoring.
Druids scored a late consolation via a deflection.
TNS: Lewis Dutton, Harry Woledge, Liam Ihekwoaba, Ismail Ozdemir, Josh Bailey, Aaron Lloyd, Ethan Short, James Parry, Owen Jones, Louis Bradford, James Rutter, Rob Johnson, Luis Edwards, Lewis Rees, Tai Jones.
The Welsh Youth Cup semi-final draw was made on Monday at the Football Association of Wales headquarters in Cardiff.
Cardiff Met and Llandudno will play their postponed quarter-final match on Sunday, February 10.
Quarter-finals: Cardiff City 10, Llanelli 0; The New Saints 5, Cefn Druids 1; Swansea City 5, Merthyr Town 1; Cardiff Metropolitan v Llandudno, postponed.
Cardiff City’s McKay twins, 22-year-olds Jack and Paul, have both gone out on loan.
Chesterfield have signed striker Jack for the rest of this season and he says: “I’m a hard-working striker and I can’t wait to get started.”
Defender Paul has joined Morecambe.
They have been playing for the Bluebirds’ under-23 development squad.