Max Dowman and Marli Salmon, both 16, were in the starting line-up, while 18 year-old defender Jaden Dixon, fresh from his move from Stoke City on deadline day, was introduced in the second half for his debut.
"Wow - at 16, they play at the highest level. And how they play - they play so well," Kepa told arsenal.com.
"They're special kids who are used to training with us, they're used to being around us.
"They had the opportunity, I think they took it, and I'm so happy for them."
Kepa also said, "The goal was to be in the next round, and we're ready. We now have the (Champions League) with a couple of big games, and we'll be focused.
"We knew a little bit these types of games and competition is always difficult. It was tricky today, we were focused and we knew what was going on. So, yeah, the most important thing is that we're through.
"I think we were expecting that. We have to play these types of games - they're not easy. The pitch, you have to adapt - there are no excuses. We had to adapt and win the game."