I can understand your concerns.
nodes are quite common in slightly bigger children. But supra clavicular one is not so common.
Hard consistency is also a matter of concern.
You have not mentioned what vaccine
was given for your kid. Yes some vaccinations can produce lymphnode enlargement.
You may try a course of antibiotics for about 2 weeks to treat any occult infections after discussion with your doctor.
You may have a complete blood count
and a peripheral smear till you have a follow up.
If the size remains the same or increases in size, detailed work up will be needed.
Don`t take much worries and hope for good.