My 3 yr old son has a palpable cervical lymph node since october,which i felt accidently, 12-13mm approx ,no other symptoms,mobile,pedi not worried.

Have it checked: 3 months is a long time for a lymph node to remain swollen. If it is a lymph node, some other process is underlying, but the chance of another process causing a lump should be evaluated.

Rarely a problem: These nodes are easily found because they are not hidden within the muscles/soft tissue. They swell while working and generally shrink back to pea sized in 3-4 wks unless there is ongoing issues. Neck nodes can reflect tonsil, throat,or skin surface, a systemic virus/vaccine. If they grow in size to walnut size, become red or painful they need special attention. They rarely mean anything else