There is no resort-wide policy, so it would be up to the restaurant. There may have been a few reports that sharing is disallowed at certain a la carte restaurants. I'm unable to remember which restaurants. But ask when you arrive. Most restaurants at WDW appear to permit and even encourage sharing.
The only certainty is that sharing will not be permitted at buffets and all-you-can-eat restaurants. Nor is it permitted to share the all you can eat skillet at Whispering Canyon.
If there is no separate children's menu at a CS location, a child's credit may be used for anything from the regular menu.