Apparently one of the victims even asked a passer-by for help but the adult ran away.
The horrible robbery, which happened in south-east London, lasted for about five minutes and was only broken up after pizza shop staff saw what was happening.
The children are said to be "very shocked and distressed".
Police, who have descriptions of the gang leaders thought to be about 14 or 15 years-old, said: "This was a very unpleasant experience for a group of children attending their local cinema. They were in a large group and obviously expecting safety in numbers."
But latest figures show that half of all street crime in London now involves mobiles.