My 3 year old son has a palpable cervical lymph node which measures 13×6mm with intact hilum got neck ultrasound done which reveals few intraparotid lymph nodes on left side with largest 5mm, thyroid and submandibular and right parotid glands normal.

Sounds pretty normal. Lymph nodes are the bodies serum filters.They remove random viruses/debris & some bacteria from the serum floating between cells. They enlarge when working & shrink when inactive over several weeks. A 3 yo will likely experience throat/ skin/ upper airway issues that trigger some gland action every so often.This study sounds pretty normal. It can serve as a baseline while you monitor progress.
Lymph node. The lymphatic system as as a filtering system for infections, trapping pathogens to keep them from spreading.Cervical lymph nodes filter the entire he'd/upper neck area, so an URI, sinus infection, or enlarged tonsils/adenoids, can trigger node enlargement. So can decayed tweet, gum disease, or one of the many transient oral infections. Pro baby would be wise to have son's Dentist check.