United's U18s slipped to a narrow defeat in Wales at the weekend.
A goal late in the first-half proved to be the difference in a game of a few chances, as the Blades suffered a second consecutive loss, which is a blow to their hopes of a top two finish.
Reflecting on the defeat, Coach Travis Binnion remarked: "We are disappointed to lose the game because there was very little between the two sides.
"We were better defensively then we were against Crystal Palace, but we lacked that spark up front and didn't create enough, which is unlike us.
"But we are still in a good position and we must look to get back on the winning trial and ensure we finish the campaign strongly as we look to secure a play-off spot."
Cardiff had enjoyed the better of the first-half, despite Crofts having a chance for the visitors and duly took the lead in the 36th minute.
The visitors almost responded with the final action of the first-half, but Smith hit the post when he should have scored.
The second-half saw goalscoring chances at a premium at both ends, although substitute Maw had strong appeals for a penalty waved away, prior to Hallam having a goal bound shot blocked.
Blades: Perryman, Lowe (Maw 45), Semple, Cummings, Gordon, Crofts, Whelan, Ford (Slater 66), Hallam, T. Smith (Graham 73), Litchfield.
Unused: Warhurst, Ramsey.