Nathan Holland scored the only goal of the game.
U18s regular Harrison Ashby joined 15-year-old Jamal Baptiste and 17-year-old Keenan Appiah-Forson in a youthful-looking defence.
Halajko told whufc.com: “It was a really tough game, probably one of the hardest ones we've had, that we edged tonight.
“It was a young back four for us - a different back four. They haven't played together as a back four at all, especially not at this level.
“Reece (Hannam) had been ill the past couple of days, so we were just monitoring how he was throughout today, hoping that he could make it, and unfortunately he couldn't, so we brought in Harrison who's just come back off international duty to play in there, and Jamal, who's only 15-years-old and played centre-half alongside him.
“Harrison normally plays right-back, Keenan normally plays centre midfield, Jamal hasn't played at this level before, so for them to gel together today and keep a clean sheet is a massive achievement."