18 month old with croup. Wakes up screaming/crying non stop, throwing himself around like a tantrum. Breathing fine. Steam not calming. Er or wait?

Sleep disorder. If your daughter does not wake immediately when you touch her and has no breathing difficulty, she is in the midst of a night terror an anomaly of sleep like sleep walking. If she wakes immediately it is a nightmare she will remember specifics.Night terrors occur in deeper sleep kids have no memory of them and often reflect sleep deficit. More sleep over several days will fix it.
Croup. You probably should have a nebulizer with albuterol at night for his nocturnal symptoms. He may be throwing a tantrum as he is not breathing right, although you can't tell. You should see the pediatrician as soon as possible and get the child evaluated.