Five of the victims -- three adults and the children -- were travelling together in the car, a Kia, which had a French numberplate according to reports.
The crash happened after a lorry shunted the car, sending it into the back of the tanker, which exploded.
The lorry driver was killed, while the tanker driver escaped alive, reports said.
Police footage showed firefighters tackling the vast blaze on the motorway in Brescia, hampered by fierce flames and a column of thick black smoke which could be seen for miles.