Most dangerous sign in LRT1 in children is:
Grunting

Grunting is an expiratory sound made by infants as they exhale against a closed glottis.
It is an attempt to increase functional residual capacity and lung volume.
This is done in order to raise functional residual capacity above the critical closing volume and to avoid alveolar collapse.
This physical finding represents impending respiratory failure and should not be overlooked.
Abdominal breathing
Chest retraction
Grunting
Tachypnea