Theoretically it's possible, but unlikely. current lore (such that it is) seems to indicate humans can be born post-collapse with the capacity to bear the Traveler's light. Shinobu and Shin Malphur are the two we know of that were born normal humans and later became guardians. In Shinobu's case, we know that she was running around with some guardians when one of the ghosts told her she carried Light, but it wasn't until after Shinobu was killed in combat that a ghost resurrected her as a Guardian. We don't know anything about Shin Malphur's transition from human to Guardian. So, I'd say it's possible for there to be child guardians, but I find it unlikely. Mainly because logistics. We're basically zombies. There's nothing to say that if a child were brought back as a guardian, that they would continue to grow into adulthood physically. They might (probably) forever remain a child. This is bad because children don't make great soldiers. Guns are all too big for them, they're not physically fit enough to fight, etc. A Ghost would probably recognize this and only search out suitable adults to rez. EDIT: further, Jaren Ward's ghost is the one who realized that Shin Malphur carried Light, and Shin was just a boy at the time. He grew into adulthood before becoming a Guardian though. At least, that's the common interpretation of that part of the lore. It's not entirely explicit.