Fitbit Ionic "Stores heart rate data at 1 second intervals during exercise tracking and at 5 second intervals all other times" - https://www.fitbit.com/shop/ionic#specs
I don't know how much useful information you can get from the variability of a metric you only sample every 5 minutes.
The parent's description isn't 100% accurate but it's accurate enough. The Apple Watch tracks every 5 minutes unless it detects an elevation in heart rate. If it detects something higher than 10 or 20 bpm, it polls more frequently. The standard, resting heart rate polling is at 5 minutes which, according to everything I've read, is more than accurate enough for most medical uses.
