Hey Elizabeth,

when their noses are blocked up, get your kids to do a gentle exhale (through the nose) and then hold their nose (and breath) for as long as they can without gasping at the end. Then, when they let go, to breathe gently through the nose. It will unblock through this process (it works really well). May take a few goes.

The Buteyko instructor also suggested nodding head back and forth (ie nodding) while holding breath/nose. Seems to increase the effect.

Good luck.