Ferry terminal security personnel called the emergency services after finding the 26 migrants in the back of a refrigerated lorry at Loon-plage, near Dunkirk, on Saturday morning.
The child was found to have a mild case of hypothermia and was sent to hospital along with the mother, an Iraqi.
Migrants frequently try to cross the Channel to Britain from northern France.
There is reported to be an increase in attempts to travel by refrigerated lorry.
Seven migrants - Iraqis, Iranians and Afghans - were hospitalised on Thursday after a driver found them in the back of one such truck travelling on a motorway in central France.