Set syncing time?

You cannot schedule it, but the syncing happens automatically, or can be triggered manually. See the details in the article How Do I Manually Sync my vivofit Jr. Device? | Garmin Customer Support (it applies generally, also for other similar devices, not only to Vivofit Jr)

vivofit jr. watches will periodically send data automatically to the nearest paired Bluetooth® device after a certain amount of steps are taken as well as after a timed activity or when a goal is met. You can also manually send data at any time by following these steps:

1. Bring the device near your smartphone or tablet.
2. Open the Garmin Jr. app.
3. On the vivofit jr. device, hold the key until you see the wifi icon icon.
4. Wait a few seconds until the red move bar icon begins to slide back and forth at the top of the display.
5. Once connected, circular arrows will appear on the device indicating that data is being uploaded.
6. From the app, swipe down to refresh data.