As a paediatric physio, having been around for a while, I have never had a child come in with such a complaint.

And, is there a complaint? What is the real problem?
Does the child have any problems?
I can imagine that her parents are worried a little...
What do they percieve as the problem? What is their biggest concern?

You say the bone seems to be limiting her ROM partially. Could you be more specific?
Is it hurting her in rest or only after your handeling?
I think that your action of sending them back for further assessment is a very good idea.

I wouldn't surprised, that if it is a bone asymmetry, there is nothing you can do.
I have seen children with asymmetric cervical ROM because of spinal deformities and skull deformities. In these cases treatment
was deemed necessary only if the child had some kind of trouble as a result.

Keep us informed please.

Esther