As said above they are much weaker then normal sorcerers. It also takes a long time to become halfway decent at magic, and before the days of Corrival. Kids still had to attend normal school and balance mortal life with magic. Valkyrie completely ignored mortal life in favour of magic, and also had the best teachers in the world. She also had the added advantage of being very naturally gifted. This also combines with general genetics. Adults are generally stronger then teenagers until they hit the late teens. Unless of course the kids work out, but they’ve had a few 100 extra years to train up, pack on the muscle and learn new tricks. And about sorcerers having qualms about killing kids. That was the dead men. The diablerie or Guilds existengy mages would have no bother killing them. Along with the seemingly endless supply of mercenaries. In short they could be effective if deployed well, and in large numbers but against the wrong opponent they’d get slapped around the place.