Trowa kept restraining him, but shifted the grip so that Ehmoi was... well, not in a position Trowa would use to slam a suspect into, say, a wall. Or the floor.
Meaning, his back was against Trowa's body weight with his head tucked forward so he couldn't try to spear Trowa and his wrists were both caught in one hand so that the kid couldn't flail while Trowa stroked his hair with the free hand.
Stationary, secure, and safe. Reassuring, repetitive movements. Warmth. It worked on mother-rejected lion cubs and dogs, surely that meant it'd work on a kid.