Following last weekend's 1-0 success over Arsenal, the young Foxes' maiden victory of the 2019/20 season, confidence was high as they headed to Beaufort Road on the south coast of Wales.
However, with just 17 minutes on the clock, City's youngsters were behind as Swans midfielder Bradley Gibbings latched onto Adnan Moti's pass to convert past American goalkeeper Chituru Odunze.
Leicester went into the break in a commanding position, though, as a fine effort from Pennant, followed by an Arlott-John brace, one of which came from the penalty spot, made it 3-1 at half-time.
Arlott-John turned provider after the interval, meanwhile, laying it on for Ethan Fitzhugh on 64 minutes before also setting up Pennant for his second, sealing a brilliant 5-1 victory for the Under-18s.
As a result, the young Foxes leap up to sixth in the South standings of the U18 Premier League pyramid with eight points from six matches, one point behind Tottenham Hotspur.
Following a two-week break, City's youngsters return to action against third-place Chelsea, level on points with leaders Fulham, at Belvoir Drive on Saturday 5 October (12pm BST kick-off).
The details...
City: Odunze; Gbadebo, Sams, Aisthorpe, D. Gyamfi; Pennant, Fitzhugh, Leathers (Tavares 19'), Loughlin; Arlott-John, Ewing (c) (Russ 80')
Subs not used: Bosworth, Flynn, Murch
Goals: Pennant 21', 79', Arlott-John 32' (pen), 41', Fitzhugh 64'