They might already be eating the small ones.
Look for them doing a hopping movement, this is them feeding.
Its not as easy to tell if they're eating with daphnia, with baby brine shrimp, their bellies go bright orange so its easy to see.
Can you get hold of some walter worms, or brine shrimp eggs a a back up food if the daphnia doesn't work??
Some people have had sucess with boiled egg yolk, theres a thread you can search for.
Mel