As you said, Family Sharing would need iOS 8 on both iPads. You can go to the App Store and click on each app in your purchases, to check that each of your necessary apps are compatible with iOS 8, but the vast majority will be so that should not be too much of a concern. The easiest way for you to carry out this entire operation is through Family Sharing, once both iPads are on iOS 8, all you would have to do is go into the App Store from the new iPad and click "purchases" which will now show your name as a family member and allow access to all your previously purchased apps that allow family sharing, including the games. Again, in App Store you can check that each of the game apps allow Family Sharing, by touching on the icon of the app and scrolling to the details area it will say "Family Sharing: Yes/No". It will be free to download any previous purchases to the new iPad. To answer your other question, yes you can back up two iPads to the same iTunes Library, they have different serial numbers and are recognized as separate devices, for ease of use it is recommended to uncheck automatic syncing on the new device for music, photos, and more. See below excerpt from the Family Sharing link above for your options with Apple ID's depending on your sons age:
Now kids under 13 can have their own Apple IDs. As a parent or legal guardian, the family organizer can create an Apple ID for a child and add the child to the family group automatically. Ask to Buy is turned on by default, and the organizer can also limit the content kids have access to on their devices through Restrictions on an iOS device or parental controls in OS X and iTunes.