After last weekend's drama in an injury-time 3-2 triumph away to Chelsea, this was a far more straightforward victory on home turf.
United completely dominated the first half and took a deserved 2-0 half-time lead through goals from defenders Sean McGinty and Michele Fornasier, before Tom Lawrence, John Cofie and Jack Barmby extended the scoreline after the break.
The opening goal arrived after 19 minutes when Tom Lawrence pulled the ball back after Cardiff failed to clear a corner and Irish defender Sean McGinty tapped the ball home from close-range. It's his second goal in as many games for the U18s, and in truth it was more difficult to miss.
Five minutes later the Reds doubled the lead, with another defender opening his account for the season - the fourth defender in two games to do so - and another goal invited on themselves by the visitors. The Bluebirds failed to deal with Larnell Cole's corner and Italian centre-half Michele Fornasier, who signed professional forms with United this week, was free to head the ball into the empty net.
It took United just ten minutes of the second half to find the third goal, which came from the penalty spot.
Norwegian midfielder Etzaz Hussain picked out Tom Lawrence's run and flighted a brilliant ball beyond the Cardiff defence. Lawrence was sandwiched by Cardiff's keeper and a Bluebirds defender to earn a penalty. The young Welshman dusted himself down and stepped up to take the penalty. He shaped to put his foot through it, but instead placed his shot past the deceived keeper.