We got one for ds right before his 5th birthday. It had a pediatric calibration and a mouthpiece that made it more naturally-sized for kid mouths.
We used to talk about taking a deep breath to blow air into the sails of a ship to run away from pirates or something like that. We made it a game, but gave ds a visualization.
What really took a week or so for him to get was the hard & fast blow.
On an asthma board I used to frequent, there would be conversations about how a peak flow meter only measures large airways. So, depending on how asthma is affecting your child's pulmonary system, a peak flow meter may or may not help as much. If the most constriction/inflammation is in smaller, lower airways, I think the PFM doesn't help as much.
If you've had PFTs done through the computer at your dr's office, you should be able to ask whether your child's larger or smaller airways are more problematic... PFMs are good if you're trying to get daily measurings of the larger airways.
sorry for novel, don't feel like paying atteniton to editing.