13 yr old child with swollen node above collor bone. Not painful no signs of sickness. Node is over 2cm. Should ultrasound be her doctors 1st step?

Would be reasonable. In an otherwise healthy child with this size of mass, getting an ultrasound would be a reasonable first step to determine if this really is a lymph node. Probably even before this, i would get some preliminary blood work (complete blood count, uric acid, electrolytes, liver function tests). If this really is a > 2cm lymph node, she may require further imaging and a biopsy.
Hx & Px first. The first step is an evaluation by the doc including a history & physical.Any other workup would be directed by the findings. I find ultrasounds of nodes to provide little usable data.