One of the new features for iCloud Photos/Photos.app in iOS 11 and macOS High Sierra is the ability to choose a different frame in a Live Photo as the key photo. The ability to do this is especially helpful if your kids look away at just the wrong second.
On iOS, find the photo you want to adjust.
Tap Edit in the top right corner of the screen. You can scroll through the frame options on the bottom.
Once you find the one you want, tap Make Key Photo, and then press Done.
On macOS, open Photos.app.
Choose the photo, and click Edit. Under the photo, you’ll see the various options.
Once you’ve chosen the one you want, click Make Key Photo, and press Done.
This action isn’t destructive, so you can choose a new key photo at any time.