As they are still so little I would be feeding them little and often, probably three meals a day of you can. It is rather difficult and expensive to get black worms over here.
Do you have ice blocks that go in a cool bag? Could you put the tubs in a box on top of the ice blocks? It should keep the water cool, without cooling it so much they loose their appetite. If you can do that then put some live daphnia in the tubs with them so they can eat as they wish without spoiling the water.