Once your child has finished breathing in deeply, try and get him or her to hold his/her breath as long as you can. Make it a game; do it together. Shoot to do it for a count of 10. If that's not possible, though, just go as long as you can without your child getting anxious.
Then, let your child exhale loudly and slowly. You could pretend you're trying to blow out a candle or a feather.

