If you want to Sync your Child Sites from just one group at once, there is an easy way to do that:
1. MainWP > Sites > Manage tab
2. From Select a group drop down menu, select which group you want to Sync:
3. Than click the Display button and your Child Sites in that group will be shown:
4. After that you need to check the checkbox to select all sites and in drop down menu select option Sync:
5. The last step is to click Apply button and your Sites will be Synced.