If they don't play louder, keep doing it until they do. "More." "More." "MORE."
"That's piano, until I say otherwise."
MAKE them use more air. There's no trick to it. They need to blow harder. Every time they are playing tiny, cut off, say "Clarinets are too soft" and do it again.
That being said, what kinds of reeds are they on? If they're too soft, they may be afraid of squeaks or distortion if they blow with good air. If they're too hard (or they're too dry), they may not be able to get the air through the instrument.