Most TS restaurants where you order a la carte from a menu you can share (i.e. use one dining credit to feed two or more people). One exception, as you mentioned, is Via Napoli where you cannot share when using credits and you cannot combine credits for the larger size pizza.
You cannot share and everyone 3 years and older will be charged at buffets, all you care to eat family style restaurants (e.g. Garden Grill), fixed price meals (e.g. dinner at BOG, Akershus, CRT, Storybook Dining), the dinner shows (Hoop De Doo, Poly Luau), all you care to eat entrees (e.g. the platter at LTT and Whispering Canyon), and special dining packages (e.g. Candlelight Processional.)