Because it isn't. Childermass can't guarantee anything. They may survive, they may get crushed. He's no warrior. If they wanted fighters from the Winter Court, they would have had to bring his remaining sisters and, well, that's not really advisable.

"Just know that I can't move you while you're holding iron," is the only warning Yuya gets before they're dropping back into shadows and moving again, past the elemental barrelling down on their previous location. Then, they're back up on the grass again, almost exactly where Yuya had been originally.