welcome to HCM!
The vast majority of cystic lesions in infants and children are congenital or developmental in origin.Swelling under your chin seems to be a congenital submental cyst.Common cystic neck masses in Submental region are Dermoid/epidermoid,Ranula
and in Midline/thyroid
region are Thyroglossal duct cyst,Colloid and haemorrhagic thyroid cystic nodule
You can get an ultrasound
done.Ultrasound also has the unique advantage over other imaging techniques in providing reliable, real-time guidance
for fine-needle aspiration
Your treatment will depend on exact diagnosis of the lesion,so get the tests done.
hope this helps!Take care!