Brief Answer:
Get coagulation profile done

Detailed Answer:
Hello

Thanks for asking on this platform


I have gone through the details and do understand your concern.
These marks can be due to unnoticed trauma or even spontaneous.
The most common cause of appearance of such bruise like hickey marks are some clotting abnormality.It may be due to deficiency of platelet or any other coagulation factor.
Here I would like to get platelet count and coagulation profile(Prothrombin time,Activated partial thromboplatin time,Bleeding time and Clotting time)done.
Depending on these investigation results further investigation may be needed to find out the underlying cause(like deficiency of coagulation factor,any liver problem etc.).
These marks are not related to thyroid disorders.
Skin marks are better diagnosed by direct visualisation.So I would request you to attach an image so that I better able to help you out.
I suggest you to get her evaluated by a pediatrician and to assess the need of these investogations.
Please let me know if you have any more query.
Regards

---

Brief Answer:
This appears to be more like papular urticaria

Detailed Answer:
Thanks for follow up and also for attaching the image

I have seen the image.The lesion has dark centre with peripheral inflammatory redness.This is more suggestive of Papular urticaria(a kind of allergy) or some other allergic reaction.
The skin lesion due to coagulation abnormality looks more purple/dark without such redness.
I would like to know whether there is any associated pain or itching.
Allergic skin lesion may appear and resolve on its own.
Application of Mild steroid helps in this condition.
Avoidance of known allergen if any and insects helps in recurrence of these kind of lesion.
Again I would suggest you to consult a pediatrician or dermatologist and then management accordingly.
I shall be glad to answer if you have any doubt.

Thanks and regards

---