Thank you for your query,
The following investigations to carried out:
Karyotyping : to diagnose down syndrome by blood investigation
function test: to detect associated hypothyroidism
2D Echo: to rule out any associated underlying cardiac defects.
X-ray neck can be done later to rule out any atlantoaxial dislocation.
Vision and hearing to be assessed.
So I advise you to consult a Pediartcian to plan for the investigations and management including genetic counseling
Hope I have answered your query.