We tried with my 9 year old when he was about 6 and he just didn't get it -- I think. The reason I say I think is that we got the same exact reading every single time. You know how they say take 3 measurements and use the best -- they'd be identical every time, and then the next day would be the same, and if he was in the middle of an attack -- still the same. The readings were also all alarmingly low, in the "red zone", so it was scary until I decided it wasn't working.
So, either we had a broken meter or he didn't do it right. I thought it was weird that he couldn't do that but had not problem with PFTs in the clinic.
Having said that they cost like $20 -- so why not get one and try it? My son's asthma is very chronic, and he has no sudden flares, so I didn't feel it was an important tool for us anyway, but if your DC's profile is different then it's probably worth really working at learning how to use it.