The ten-month-old baby was left in a critical condition and flown by helicopter to Lisbon after suffering cardiac arrest following the accident, but later died.
A 14-year-old girl has also been taken to hospital with injuries but survived the ordeal.
The cart was carrying a gypsy family of six on the EN2 when due to its poor visibility it was struck by two cars.
The drivers of the vehicles, who suffered just light injuries, had to be escorted by police from the scene after tensions flared when relatives of those killed in the accident arrived.
The horse and family’s dog were unscathed in the accident.