Cody Twose opened the scoring in the Welsh capital, before a Simic brace and a stand-alone strike from Cole Fleming sealed the victory for the home side.
Both teams went into the fixture vying for victory – albeit for different reasons; the Bluebirds were looking to break their seven-match winless run, while our youngsters were looking to build on their recent successes.
City set the tone early on, as Tanatswa Nyakuhwa was found in a pocket of space, feeding the path of TWOSE who rounded Bastian Smith to draw the first blood with nine minutes on the clock.
Bloxham’s men scented a quick second, Raheem Conte – who proved a real outlet over on the left side – sent a ball in towards Nyakuhwa, whose effort was well met by Smith.
The Bluebirds continued to set their stall out, this time Dakarai Mafico landed on Smith’s errant pass, who quickly made amends by tipping Troy Perrett’s strike onto the woodwork.
Nyakuhwa looked to land the next telling strike, driving forward with Simic in support to his left – but he opted to avoid his counterpart, forcing Smith into a smart stop low-down to his far post.
Our youngsters discovered newfound attacking intent late in the half, with Andre Changunda and Joe Davies trying their luck – with both efforts clearing the upright.
Failing to capitalise, the hosts worked the ball from back to front, with SIMIC wrongfooting Smith at the other end to round off a neat City move.
Claxton looked to reduce the arrears, rising above his marker to meet Davies’ delivery – finding the back of the net. In the end, the referee ruled the goal out, claiming a City man was impeded in the build-up.
The second period followed a similar pattern as it did an hour earlier; City were quick out of the traps, as Simic’s timely run abled the front man to square Nyakuhwa, who miscued his strike.
Looking to soak up the pressure, Eddie Fox pushed for a way back on 66 minutes, picking out Patryk Sykut in space, whose driven strike was thwarted by the woodwork.
Ma’Kel Campbell followed suit, standing up to a dead ball, with the woodwork coming to the host’s aid once more.
Max Beech entered the frame soon after, looking to make an instant impact as he curled an effort towards the top corner – narrowly missing the target.
The hosts pressed for a third, and it came with a quarter of an hour left to play, as SIMIC arrived on cue to flick home Will Spiers’ delivery.
FLEMING added a fourth – and put the game out of sight – five minutes later, picking out the top corner from 25-yards on his return from injury.
Posh: Smith, Claxton (sub Trialist A 72min), Fitzpatrick, Campbell, Gilbert, Mendonca, McWilliams-Marcano (sub McWilliams-Marcano 80min), Fox, Davies (sub Beech 70min), Sykut, Changunda. Unused subs: Aikins, Westcott.
Half-time: Cardiff City 2-0 Posh Under 21s
Full-time: Cardiff City 4-0 Posh Under 21s